Observations of strained nanocrystalline materials have shown that discontinuous grain growth can occur at room temperature with as little as 2% strain [1]. Understanding such changes in nanocrystalline material properties at room temperature is vitally important and has, until now, required detailed TEM observations of the resulting microstructure and inferences regarding the processes involved.

Transmission Kikuchi diffraction (TKD) in the scanning electron microscope (SEM) has advantages in resolution over conventional electron backscatter diffraction that have resulted in a wide range of applications of TKD across the materials and Earth sciences [2]. However, to date TKD has not been applied to the study of dynamic processes, utilizing in-situ deformation or heating. Here we present results from a prototype in-situ tensile deformation stage that has been custom modified for use with TKD, enabling the investigation of discontinuous processes on the nanoscale, such as stress-assisted grain growth.

Nanocrystalline copper and aluminum samples were deposited with a range of grain sizes and thicknesses, and these were then separated from their substrate and floated onto TEM-compatible push-to-pull (PTP) holders (Hysitron Inc., USA). A focused ion beam was used to shape the film into a suitable dog-bone configuration, so as to enable analysis in transmission. The PTP holder was then mounted on a custom-modified Hysitron PI-85L in-situ nanoindentor, designed for tensile testing of thin film samples with simultaneous TKD mapping. The experiments were carried out using a Zeiss Ultra Plus field emission SEM equipped with an Oxford Instruments AZtec EBSD and TKD system. Tensile tests were carried out using standard TKD conditions (typically 30kV accelerating voltage, 5-10 nA beam current, step sizes ~ 5 nm and mapping speeds ~ 66 Hz), with loading up to ~ 500 uN. In all cases in-chamber plasma cleaning enabled multiple scans of the same area of the sample by minimizing contamination. A nanocrystalline Cu film with a bimodal grain size and a thickness of 55-75 nm was deformed up to failure (at 450 uN) and mapped 8 times using TKD at different loads.

The results show the potential of this in-situ approach for the characterisation of nanostructural changes throughout a deformation experiment using TKD. In this example we show how the TKD mapping was able to track the (lack of) changes in the grain structure and the eventual failure. The system was sufficiently stable to enable accurate quantification of nanoscale changes.

We believe that this in-situ TKD technique can become an effective tool for understanding the complex nature of discontinuous stress-assisted grain growth, allowing us to identify the exact nature of the grains and boundaries that are most affected by this mechanism.References:[1] D.S. Gianola et al., Acta Materialia 54 (2006) p. 2253–2263.[2] G. Sneddon et al., Materials Science and Engineering R: Reports, 110 (2016), p. 1-12.

8:30 AM - *TC06.01.02

The Utility of DFT Simulations in Rationalizing Slip in Complex Crystals

The development of small scale testing, which is now able to test different phases of multiphase materials, as well as the interest in new material development has led to a need to describe and understand slip in materials that have more complicated bonding than metals and their alloys. Due to the complex bonding that is likely present in such materials, DFT has become a regularly utilized tool investigate dislocation slip most often through the computation of generalized stacking fault energy curves. In this talk, we will present some case studies where DFT simulations are used in this way investigate dislocation slip at the smallest scales. In certain cases, the role of chemistry and bonding, beyond simple interatomic spacing, is clearly important and can only be explained using DFT while in other systems, more simple models suffice. A number of interesting material systems including metal carbides, intermetallics, and unique ternary compounds will be presented. These cases studies will then be compared directly with experiments, either providing insight into the experimental results or providing additional insight into observations that were unable to be made in the experiment.

9:00 AM - TC06.01.03

Superelasticity and Micaceous Plasticity of the Novel Intermetallic Compound CaFe2As2 at Small Length Scales

Shape memory materials have the capability to recover their original shape after plastic deformation when they are subjected to certain stimulus. Shape recovery usually occurs through a reversible phase transformation and, in general, has limited performance with 10% maximum strain. Here, we report the first discovery of superelastic and shape memory behavior with 13% recoverable strain in a novel intermetallic compound CaFe2As2, and discuss its unique elastic and plastic deformation behaviors in terms of a collapsed tetragonal phase transition and anisotropic stacking fault energy, respectively, with solution growth of the single crystal, in-situ micropillar compression, and density functional theory (DFT) calculations.

Single crystals of CaFe2As2 were grown out from Sn flux and contains mirror-like clean facets of {0 0 1} and {3 0 1} type planes. We fabricated micropillars on these two planes, and conducted in-situ micropillar compression test in a scanning electron microscope. CaFe2As2 exhibits unprecedented superelasticity: over 13% recoverable strain without any residual fatigue damage under cyclic deformation. The [0 0 1] CaFe2As2 micropillar also has yield strengths over 3.5 GPa at room temperature, and has potential to show one-dimensional shape memory effects at low temperatures (near 0 K) by the reversible phase transformation between tetragonal/orthorhombic to collapsed tetragonal phase. Also, this material exhibits strong anisotropy in plasticity. For [3 0 1] CaFe2As2 micropillar, we found easy, preferential slip in the [1 0 0]/(0 0 1) slip system which we termed micaceous plasticity. Superelasticity and micaceous plasticity was quantitatively investigated through measuring the uni-axial stress-strain data and comparing our results to DFT calculations. DFT calculations revealed that making and breaking As-As bonds is responsible for superelasticity. A composite model was developed to monitor the volume fraction evolution of the two different phases under compression testing and successfully reproduced the experimental stress-strain curve we measured. In addition, DFT results showed a significantly low energy barrier for [1 0 0]/(0 0 1) slip between Ca and As layers, which agrees with our experimental observation. We believe that our efforts in both experimental and computational analysis allows us to gain a fundamental understanding of the unique deformation behavior of CaFe2As2.

9:15 AM - TC06.01.04

Microstructure Changes Due to Heat Treatment and Their Effects on the Plasticity of Aluminium Alloys

For decades, aluminium alloys have been known for their light weight and low temperature use. Thanks to this, today they are widely used for structural components but manufacturers are still searching for ways to reduce the aircraft weight. In this study conducted by the IRT Saint Exupery with the CEMES laboratory in Toulouse, aluminium alloys are exposed at higher temperatures than traditionally to replace titanium alloys as a new approach to weight reduction.

To achieve this goal, this study revolves around a heat treatment at 200°C for 1000 hours to evaluate the changes due to this long-term exposure of a few numbers of selected industrial aluminium alloys. Literature is scarce on studies dealing with properties of aluminium alloys at this temperature and focuses only on the evolution resulting from short exposures (less than 500h). However, it has highlighted two alloys developed for medium temperatures, the 2219-T851 and the 2618-T851 alloys which were already used on aerodynamics parts at higher temperatures. Two more alloys were added following their recent development, the 2050-T84 and the 2122-T8511. A multi-scale characterisation was conducted on these four alloys through mechanical tests at room temperature as well as detailed observations with scanning and transmission electronic microscopes.

This presentation is focused on the effect of the microstructure changes on the plasticity of two aluminium alloys, the 2050 and the 2219. Indeed, the mechanical characterisation has highlighted drastic changes on the alloys behaviour after long term exposure. To understand these changes, conventional and in situ transmission electron microscopy observations were conducted.

It is well known that in nanocrystalline (NC) materials, dislocation plasticity and grain-boundary- (GB-) mediated diffusive plasticity are generally viewed as two major competing mechanisms of plastic deformation. Given the similar roles that surfaces and interfaces play for both dislocation nucleation and diffusional plasticity in nanocrystalline materials, nano-sized metals could have similar phenomena compared to those of NC materials, owing to the competition between displacive (dislocation type) and surface diffusional plasticity. Thus, the strength of small-volume materials could be generally divided into regimes, where the Hall-Petch-type relationship between crystal size and strength in the dislocation-plasticity-dominated regime converts to an inverse Hall-Petch-type relationship when plastic deformation is governed by diffusive processes in the lower nanoscale. Here, by performing in situ atomic-scale transmission electron microscopy, we report unusual room-temperature super-elongation without softening in face-centred-cubic silver nanocrystals, where crystal slip serves as a stimulus to surface diusional creep. This work provides insight into the atomic-scale coupled diffusive-displacive deformation mechanisms, maximizing ductility and strength simultaneously in nanoscale materials.

During the propagation of a lattice dislocation in a nanocrystalline structure, the grain boundary (GB) has to interact with the impinging Burgers vector. Such interaction mechanisms are usually classified as dislocation-GB accommodation and often ascribed to atomic shuffling or stress assisted free volume migration.Here we present a dislocation-GB interaction mechanism where the misfit available in a nanosized GB can assist the propagation of a lattice dislocation. The mechanism occurs at strain rates 2 orders of magnitude lower than those usually applied in molecular dynamics. An impinging dislocation with a Burgers vector unfavourable to pass the ledge structures of the GB, double cross-slips and interacts with a dislocation loop nucleated from a misfit region of the GB. This results in a collective dislocation propagation along the GB, where the Burgers vectors of the initial dislocation is spread across three planes. The mechanism is only observed at a reduced strain rate of 106 and facilitates the slip of a lattice dislocation with a low Schmid factor. Additionally, the interaction of this dislocation with a dislocation nucleated from another GB and gliding on a parallel slip plane, results in the emission of a vacancy in the grain interior. Both mechanisms do not occur at higher strain rates of 108 where high Schmid factor slip systems are preferred.The observed dislocation events are preceded and/or accompanied by several time-dependent GB accommodation processes, brought to the foreground by the reduced strain rate of the simulation, such as GB dislocation and local GB migration as well as GB diffusion.

10:00 AM -

BREAK

TC06.02: Deformation of Metallic Nanostructures I

Session Chairs

Erik Bitzek

Frederic Sansoz

Monday AM, November 27, 2017

Hynes, Level 2, Room 210

10:30 AM - *TC06.02.01

Differences and Similarities in the Mechanical Response of Nano-Objects

Nanoscale metallic objects like thin films, nanoparticles, nanowires, or nanoporous metals receive sustained attention due to their size-dependent mechanical properties, which can include changes in deformation mechanisms, pseudoelastic behavior and increased yield strength compared to the bulk material. Usually, these types of nano-objects are studied individually, with the focus on varying the characteristic length scale of self-similar structures. Experimentally, a direct comparison of the mechanical properties of different nano-objects can be complicated as different types of objects with identical characteristic sizes might not be manufacturable, and different production routes needed to produce the different structures can influence the chemical composition, surface quality and defect contents. Simulations do not suffer from these complications. However, the variability, e.g., of the critical resolved stress for dislocation nucleation or of the deformation mechanisms for different nano-objects and loading conditions has not been studied in detail.

Here, we present results of in situ compression tests, atomistic and finite element (FE) simulations on nanoporous Au and compare them with simulations on Au nanoparticles, nanowires and thin films of identical characteristic sizes. The nanoporous simulation samples were constructed using 360° electron tomography data as well as using gyroid structures. The virtual nanoparticles, wires and thin films were built by idealizing experimentally determined structures. Additionally, twin boundaries and surface roughness were introduced in selected samples to study their influence on the mechanical response. Simulations were performed using different atomistic interaction models and constitutive models and compared with experiments. The results are discussed in the context of the transferability of models for the mechanical behavior of nano-scale structures.

Structurally, nanoporous (NP) system can be regarded as a network of interconnected nanowires as its constituting ligaments. In this study, molecular dynamics (MD) simulations are employed to investigate the mechanical behaviors of single-crystalline Cu nanowires aligned in the <001>, <110> and <111> crystallographic directions under tensile loading and their triple junction network. Shear strain tensor analysis is used to differentiate deformation mechanisms accommodating strain among nanowires and capture the necking point during stretching. In addition, a computational analysis method is used to quantify plastic and elastic deformation. The nanowires in different crystallographic orientations behave differently in elongation. Nanowire in the <110> orientation shows enhanced ductility compared to the <001> and <111> directions because of the extended twining activity while dislocation glides lead to necking in the other two nanowires. In general, dislocation activity accommodate 10% to 20% plastic deformation after yielding while most of the plastic strain generates within bulk and surface atoms. We also investigated the deformation behaviors of triple junctions. We found out that yield strength of triple junction structure is quite close to the strength of NP structure. This means that deformation behavior of the NP system can be represented as a function of junctions rather than only single ligaments.

Direct in-situ observations made experimentally have recently shown room-temperature super-elongation without softening in face-centered-cubic Ag nanocrystals, accompanied by unusual slip-activated surface creep. This interplay mechanism was observed to govern the plastic deformation of nanocrystals over a sample diameter range between 15 nm and 50 nm in Ag nanowires, which not only extends far beyond the maximum size for pure diffusion-mediated deformation (for example, Coble-type creep), but also is at the extreme limit of the “smaller is stronger” trend. On the contrary, Pt nanowires were found to exhibit only low ductility and quasi-brittle fracture under same size and loading conditions. This talk will present our modeling efforts using atomistic simulations and kinetics theory to account for the material-dependent strong-yet-stretchy behavior in sub-100-nm-sized Ag and Pt nanowires. Molecular dynamics simulations show strong negative strain-rate dependence on pre-necking elongation in Ag nanowires due to enhanced surface creep near their ideal strength limit. A new mechanistic theory based on crystal-slip and surface-diffusion thinning rates will be presented for predicting the onset of plastic and diffusion-induced instability, as a function of diameter and strain rate, which explains the fundamental difference in fracture behavior between Ag and Pt nanowires. These results could set a new paradigm for understanding size-dependent strength and ductility among different nanoscale metals, and are significant for the development of nanowire networks in flexible transparent conductive electrodes in energy applications.

Nanoporous materials such as gold (np-Au) have attracted considerable attention owing to a high surface-area-to-volume ratio, leading to a wide range of potential applications in fields such as catalysis, sensing, MEMS, etc. Testing bulk samples is difficult and tedious, and therefore we have worked to probe mechanical behavior using indentation techniques. It is also more tractable to fabricate nanoporous materials in thin film form, and this approach opens up new np material systems for study. With recent advances in focused ion beam (FIB) technology and automation, 3D reconstructions are more accessible and precise, and these provide important input for modeling deformation behavior.

Nanoindentation was performed on thin film gradient and bulk nanoporous materials, followed by 3D characterization of the deformed ligament structure under these indents. Serial sectioning was performed by FIB to obtain tomographs of indented nanoporous regions, allowing us to see how individual ligaments changed orientation as they deformed. These results enable direct visualization of the propagation of deformation into the nanoporous structure, as well as interactions with the substrate for deep indents into thin films.

In addition to the insights gained from 3D reconstruction of deformed ligament structure under an indent, we will discuss the broader application of nanoindentation techniques to nanoporous thin films, and the ability to obtain maps of mechanical behavior for wide ranges of relative density for various nanoporous materials.

Nanoporous gold (np-Au) is attracting increasing attention due to its low density, high specific surface area and high electric conductivity to offer potential benefits for many applications, such as catalyst, sensor and actuator. Unlike bulk polycrystalline Au, np-Au shows brittle behavior even though individual ligaments are composed of Au with the high ductility, making it difficult to use for many applications like MEMS, so the mechanical properties of np-Au have been intensively studied. But so far, there are few studies on creep behavior of np-Au. Time-dependent deformation behavior, such as creep, can be accelerated further at the nanoscale, and studies using nanoindentation tests have been actively conducted to evaluate them. Recent studies have shown that high grain-boundary density in precursor alloys lower the flexural strength of np-Au, while nanoindentation hardness is independent of the within-ligament microstructure. Here we fabricate np-Au samples with different microstructure, grain boundary density and initial dislocation density, and measure its time-dependent deformation properties using spherical nanoindentationWe prepare annealed, prestrained, and high-energy ball-milled Au-Ag precursor alloy. Np-Au samples were made by free corrosion dealloying process which selectively etched Ag from Au-Ag alloy. Since the microstructures of precursor alloys such as crystallographic orientation and grain size are preserved during dealloying, we obtain nanocrystalline np-Au with grain size 300 nm from ball-milled precursor alloy. Nanoindentation tests were performed to investigate the effect of such microstructural variation on the creep behavior of np-Au. We used a spherical tip for nanoindentation and the results were analyzed by Garofalo’s equation to calculate creep stress exponent, which is dependent on the creep mechanism. We discuss the characteristics of the creep deformation mechanism of np-Au with open-cell structure, high initial dislocation density and grain boundary density.

Metals that are strained past their elastic limit will undergo permanent and irreversible plastic deformation. It has been recently observed that sub-10 nm metal nanocrystals that are compressed to large strains will completely recover their initial shape and microstructure upon the removal of the applied load. Here, we explore the size-dependence of this pseudo-elastic behavior by compressing colloidal Au nanocrystals in a diamond anvil cell (DAC). Spherical Au nanocrystals from 4 to 40 nm in size were compressed to >10 GPa under hydrostatic, and non-hydrostatic pressure environments. Optical absorbance was used to monitor changes in nanocrystal size and shape during compression. The plasmon resonance of Au nanocrystals is very sensitive to nanocrystal size and shape, so shifts in the location, width and intensity of the Au plasmon peak are related to nanocrystal structural evolution under pressure. Finite element modeling was used to correlate the DAC pressure environment to nanocrystal optical shifts and structural changes, and determine the stress at which the nanocrystals are deformed past their elastic limit. We find that 4 nm Au nanocrystals recover their original size and shape when the applied load is removed, even when compressed by ~20% along one direction into an oblate spheroid shape. In contrast, 41 nm Au nanocrystals show permanent plastic deformation upon compression as expected for bulk metals.

In spite of their timescale shortcoming, molecular dynamics (MD) simulations are extensively used to complement experimental approaches and successfully provide theoretical evidences of elementary plasticity mechanisms in nano-objects. Nevertheless, samples designed for MD nanomechanical simulations are regularly model-shaped. Indeed, virtual nanoparticles (NPs) and nanowires (NWs) exhibit faceted flat surfaces bounded by sharp corners and edges whilst nanospheres are generally perfectly (and symmetrically) shaped, contrary to their more random and blunt experimental counterparts.What are the consequences of such shape simplifications?Attempting to confront this fundamental question, we propose here to investigate the mechanical response and the elementary plasticity processes of blunt NPs under compression compared to their originally perfectly cubic and sharp shape using a large set of MD simulations.The L12 Ni3Al crystalline structure is used as it provides a wide selection of dislocation-based processes but results still apply to other crystalline structures.While the lack of size-effect in originally cubic-shaped NPs is confirmed, a strong shape-effect is noticed i.e. smoothing corners and edges leads to strengthening.Furthermore, while the plastic deformation of nano-objects is generally attributed to dislocation-based processes happening from the surface of the sample, here we show that homogeneous dislocation nucleation also occurs in blunt NPs giving a certain range of sizes, in a similar manner to nanoindentation.The nucleation of partial dislocation in partial slip systems with higher Schmid factors is also confirmed as well as their contribution to deformation twinning.This study emphasizes how much the design of virtual samples is crucial to model nano-objects mechanical properties using MD and provides new insights on potential incipient plasticity features at the nano-scale.

We have studied the effects of focused-ion-beam (FIB) irradiation, pre-straining, and thiol self-assembly monolayer (SAM) on the mechanical properties of Au microparticles deposited on a sapphire substrate. The Au microparticles, which were produced by a solid-state diffusion dewetting technique, were FIB irradiated and/or pre-strained, the latter using a nanoindenter with a flat-ended punch operating under a nano-hammering mode. The pre-strained Au microparticles were then exposed to FIB to examine the effects of ion-beam damage on the properties of crystals containing mobile dislocations.

Pristine Au microparticles exhibit a strong size effect for large particle size (D>300 nm), the limiting strength of (D<300 nm) corresponds to the theoretical maximum strength of Au. Transmission electron microscopy confirms that threading dislocations are equally spaced with a distance of ~200 nm, which is similar with the observed critical particle size, ~300 nm. We found that both FIB irradiation and pre-straining reduced the yield strength of pristine Au microparticles significantly and made the stress–strain curves erratic. FIB irradiation, however, does not affect the mechanical properties of pre-strained Au microparticles significantly. Once a microparticle contains mobile dislocations, its mechanical properties are nearly uninfluenced by the defects generated by FIB irradiation, even at the submicron scale. We also deposited a thiol SAM, which is known to produce a significant amount of surface stress that would affect the dislocation nucleation stress. We found that the SAM layer does not alter the maximum yield strength, but instead increases the statistical variation of yield strength. Microparticles with SAM sometimes exhibit multiple strain burst behavior during plastic deformation while a pristine microparticle always deforms with a single large strain burst. This implies that the SAM layer affects stress concentration behavior at dislocation nucleation sites, which exist at the top corner of the microparticle.To understand the size effect of the pristine microparticle, we used dislocation nucleation theory in conjunction with statistical analysis of the dislocation source. Our statistical modeling explains that the transition of the size effect is strongly correlated with statistical operation of dislocation sources within the specific spacing. Our experimental and modeling studies enhance our fundamental understanding of dislocation nucleation behavior, and will offer the various experimental methods to control the strength of metals at small length scales.

2:30 PM - TC06.03.04

Size Effect in Strength of Fe Single-Crystalline Nanoparticles under Compression

In this work we investigate the deformation of α-Fe pristine single crystalline nanoparticles and nanowires. It was found experimentally that under compression, nanoparticles deforms continuously up to compressive stresses in the GPa regime, followed by a limited strain burst [1]. In this talk we present molecular dynamic (MD) simulations aimed at gaining insight on the mechanisms governing the deformation on the atomic level. During compression, we identified nucleation of ½<111> dislocations at the top vertices of the specimen and their glide towards the bottom of the nanoparticle, until being arrested by the rigid substrate. Compressing further, more dislocations nucleate at the same nucleation sites, gliding on consecutive adjacent parallel slip planes. As a result, two independent dislocation pile-ups form inside the particle. We propose that the continuous deformation in the experiments is pseudo-elastic, during which the pile-ups are formed, with an apparent reduced effective elastic modulus. The strain burst observed in the stress-strain curves is attributed to the break-down of the pile-up, This break-down of the pile-up, named cross-split, occurs as the two leading dislocation are pushed together resulting in a splitting of the leading dislocation.To understand the effect of size on the strength, faceted nanowires of various sizes were compressed diametrically [2]. The compressive stress at which dislocations were nucleated decreased with the size of the nanowire. The stress increment between consecutive nucleations was also found to be decreasing with size. Using dislocation theory and Finite Element Modelling (FEM) we propose that the size effect corresponds to a sum of three power laws, characteristic of three different mechanisms: the stress needed to nucleate new dislocations at a point of stress concentration, the back-stress from the existing pile-up on the nucleation site and the image stresses from the pile-up in the confined volume. The proposed model is shown to predict the stress to cross-split in the MD simulations and the number of dislocations the wire can accommodate in each pile-up before cross-splitting.

The extensive ductility of nanoglasses (NGs) makes them a promising material for the design of metallic glass (MG) composites. In this work, we considered such possibility and evaluate the mechanical properties of MG-NG nanolaminate composites by performing tensile loading molecular dynamics simulations. We characterized the effects of NG layer thickness and separation as well as the loading direction on the composite strength and profile of plasticity. The results indicate that an inverse Hall-Petch relationship rather than a linear rule-of-mixtures dictate the nanolaminate strength versus MG volume fraction. Nanolaminates with NG layers separated by more than 50 nm fail by shear banding. In contrast, by closely packing NG layers, the nanolaminates exhibit superior tensile ductility regardless of the loading direction. For instance, by decreasing the distance between NG layers to 4.8~6.5 nm, a transition from shear banding deformation to nearly superplastic flow is observed under tensile loading either perpendicular or parallel to the interfaces. Our work identifies the MG-NG nanolaminate structure with NG layers closely packed and interfaces oriented parallel to the loading direction as the most effective structure, preserving the superplasticity while still producing a strength of 2.35 GPa, which is 15% larger than the strength of NG at a grain size of 5 nm. These results are expected to encourage the development of enhanced strong and highly plastic MG matrix composites.

Metal-graphene nanolayered composites were previously shown to have ultra high strength due to the ability of the high strength 2D structure of graphene effectively blocking dislocation motion. In this study, the Cu-graphene nanolayered composite with 100 nm repeat layer spacing was subjected to bending fatigue at 1.6% and 3.1% strain up to 1,000,000 cycles while monitoring the fractional change in resistance in-situ. Cu-graphene nanolayers showed 5-6 times enhancement in retention of resistance in comparison to the Cu only thin film. TEM analysis as well as MD simulations were performed to gain insights to the mechanism for the enhancement. The cause for such enhancement in robustness against fatigue induced damage is attributed to the graphene layer making crack propagation difficult while blocking/deflecting the crack at the Cu-graphene interface. Developed Cu-graphene nanolayers with enhanced robustness against bending fatigue makes this material well-suited for flexible interconnect applications.

Direct laser writing (DLW) has enabled the fabrication of truly three-dimensional (3D) nanostructures for versatile applications ranging from optics to biomedical engineering and mechanical metamaterials. DLW fabricated architected structures with feature sizes in the micro/nano scale, also known as nanolattices, have shown extreme mechanical properties which cannot be found in natural occurring materials. Such properties include negative Poisson’s ratio as well as high shear and bulk moduli. Nanolattice materials also possess extreme properties in the dynamic range and allow for the tailoring of energy absorption and ultrasonic transmission properties. Carbon nanotubes (CNTs) are well known for their exceptional electrical, thermal, optical and mechanical properties. Due to the extreme stability of the carbon-carbon bonds, pure CNTs are one of the strongest and lightest materials known and have inspired numerous applications. Recently it has been shown that CNTs can be embedded into photoresist and structured using DLW to fabricated stable and electrically conductive composites. Within these composites the writing method aligns CNTs within the matrix, which has a dramatic influence on the electrical conductivity.Here we study the mechanics of DLW fabricated nanolattices reinforced with multi-wall CNTs. The incorporation of CNTs in the polymer nanolattices causes a significant reinforcement on the mechanical stability and properties of the composites, which can be tuned by varying the truss thickness of the lattice. The selection of appropriate surface functionalization for the CNTs enables a good adhesion between the filler and the polymer matrix, while employing DLW as a fabrication method for these composite materials allows for CNTs alignment within the polymer trusses. The good adhesion and alignment of CNTs lead to a successful reinforcement on strength and stiffness of the lattices which becomes more significant as the truss thicknesses decrease. This “smaller is stronger” effect has been observed in ceramic nanolattices but here we explore and report it for composite materials for the first time. We show that the ratio between characteristic nanoparticle size and structural dimension can significantly impact the mechanical response yielding a six-fold increase in effective Young’s modulus and a doubled effective strength in the studied composite nanolattices.

The enhancement of tribological interactions—friction and wear—of surfaces in contact is a significant challenge. A monatomically thin layer of graphene deposited onto a surface is known to reduce friction significantly, but graphene is a brittle material so its wear properties are limited. This study investigates the friction and wear of a lamellar metal-graphene-metal composite. We demonstrate a transfer-free material design comprising of monolayer CVD graphene sandwiched between a Cu substrate and a thin Cu film deposited via physical vapor deposition (Cu-Gr-Cu). A series of scratch tests performed on the free surface of the Cu film shows a considerable decrease in coefficient of friction from 0.4 to ~0.12 and an improved wear resistance for the Cu-Gr-Cu laminate. The reduction in coefficient of friction is proposed to be associated with a lower degree of plastic deformation in the Cu-Gr-Cu laminate suggestive of graphene’s ability to impede the propagation of the plastic zone from the copper film to the underlying copper substrate. Transmission electron microscopy images confirm the effectiveness of graphene in blocking the propagation of the plastic zone.

Membrane separations play an important role in the mitigation of global problems, such as water shortage, or air pollution. Nanoporous graphene membranes have significant potential to advance membrane technologies for gas separation, water desalination, chemical separation and nanofiltration. Understanding the mechanical strength of porous graphene is critical because membrane separations often involve high pressures. We studied the burst strength of CVD graphene membranes placed on porous supports at applied pressures up to 100 bar by monitoring the gas flow rate across the membrane as a function of pressure. Increase of gas flow rate with pressure allowed for measurement of the fraction of graphene that failed under increasing pressure. SEM and AFM images acquired before and after the burst test were in good agreement with the gas flow rate measurements. Consistent with theory, the membranes were found to withstand higher pressures when placed on porous supports with smaller pore diameters, but failure occurred over a surprisingly broad range of pressures, attributed to heterogeneous susceptibility to failure at wrinkles and defects and slack in the suspended graphene. Remarkably, non-wrinkled areas withstood pressure exceeding 100 bar, at which many kinds of membrane suffer from compaction. As an essential aspect of the graphene membranes for separations, the effect of creating a high density of sub-nanometer pores in graphene on its mechanical strength was also studied. We find that these nanoporous graphene membranes are still ultra-strong, although there is a finite decrease in the ability to withstand pressure. Our study shows that single-layer graphene membranes can sustain ultra-high pressure especially if the effect of wrinkles is isolated using supports with small pores, and suggests their potential for use in high-pressure membrane separations.

4:30 PM - TC06.04.05

Slip-Induced Bending Stiffness Change in Two-Dimensional Materials and Its Heterostructures

The bending stiffness of a given material plays a large role in the mechanism of structural deformation in many advanced electronic devices such as flexible electronics. While the scaling laws for bending moduli in thin films are well understood in continuum mechanics, there is still controversy about how these laws may break down in the molecular regime.The emerging class of two-dimensional (2D) materials are excellent candidates to explore the mechanical behavior of solids at the molecular scale. Research of 2D materials typically focuses on the electronic properties of stacked layers, and there are comparatively fewer studies on the mechanics of van der Waals interfaces. It has been previously reported that 2D materials violate scaling laws of continuum bending stiffness models [1,2] in multi-layered 2D materials, necessitating an investigation not only into multi-layered 2D materials, but also into heterostructures composed of them.Here we present an analysis of bending stiffness in multi-layered 2D materials using both ab-initio simulation and experiment. We use density functional theory (DFT) with various rippled 2D structures to calculate bending stiffnesses for different stacking configurations. Our simulation demonstrates the bending modulus of 2D materials is linearly proportional to the number of layers in the system, in contrast to the conventional continuum model that scales cubically with thickness. Based on a structural analysis of the simulated systems, we find that the deviation under large deformation arises from slip between layers and the formation interlayer dislocation-like structures.In order to test the results of the simulations, we fabricated rippled 2D materials and heterostructures by sequentially transferring monolayer graphene and molybdenum disulfide onto a pre-strained PDMS substrate, and then releasing the strain. We then analyzed the rippled structures using atomic force microscopy (AFM) and cross-sectional transmission electron microscope (TEM) images. Finally, we compared the shape, wavelength and amplitude of the ripples in various the rippled 2D structures to the DFT simulation.

The evaluation of the effective behavior of a heterogeneous material within a matrix has been one of the ultimate goals of the studies on composite micromechanics. Several theories have been developed over the past decades where the individual materials are treated as a continua via continuum mechanics and the ultimate properties of the composite material are strongly affected by their individual properties and arrangement. However, the randomness of each individual system and the properties of all the components of the system along with the geometrical characteristics, the formation of an interphase and the orientation or distribution of the filler within the matrix play a major role that need to be reflected on those theories.In this work we have taken into account the well-known shear-lag model for studying the behavior of discontinuous nanoplatelets in a matrix, along with the modified rule of mixtures which also includes important attributes of flakes within a composite, such as orientation and length effects to develop a universal micromechanical theory. The concepts of the effective modulus (Eeff) and the intrinsic modulus (Ef) of the filler were introduced as they play a major role on the ultimate reinforcement of the composite. Moreover, since the mechanical properties of composites can be evaluated by both tensile testing and Raman spectroscopy, the theory was adjusted to the fundamental equations that describe the results from both techniques.The results have shown that the developed micromechanical theory can provide information on the dependence of the relationship between the major parameters that are known to affect the ultimate properties of nanocomposite materials. Rather surprisingly, it was found that Ef is independent of the Young’s modulus of graphene (or other 2D-materials) and depends principally upon the second power of the aspect ratio, s (Figure 1). In addition, the t/T ratio, which can be considered as an indication of the quality of the formed interface between the matrix and the filler, is also very important for the evaluation of the reinforcement efficiency. Additional parameters such as Eeff and Krenchel orientation factor ηo have been also taken into consideration. The theory has been applied to a number of experimental results obtained from graphene-based composites consisting of matrices of various stiffness ranging from soft elastomers to thermoplastics and thermosets. The good agreement between experimental and theoretical results indicates that the proposed theory can be successfully applied for the evaluation of the mechanisms of reinforcement from graphene nanoplatelets or in general, platelet-like fillers produced by 2-dimensional materials.

Au materials have attracted much attentions for the application in micro-electrical-mechanical-system (MEMS) accelerometers to replace the conventional Si-based components [1]. High density of Au (19.30 g/cm3 at 298 K) allows further miniaturization and sensitivity enhancement in the MEMS device due to suppression of the Brownian’s noise. However, Au is known to be a soft metallic material. The yield strength of bulk Au is reported to be 55–200 MPa, which is much lower than the fracture strength of Si materials (1–3 GPa) [2]. Low mechanical strength in Au becomes a concern on the structural stability for applications as movable components in MEMS devices.There are several mechanical strengthening mechanisms for metallic materials. In the case of electroplated metals, grain boundary hardening [3] and solid solution hardening can be readily applied because of the ease in controlling the grain size and the composition by adjusting the electroplating parameters. In this work, we present the achievements obtained in strengthening of electroplated Au materials by grain refinement and alloying with Cu. For applications in MEMS, the specimens evaluated were micro-pillars fabricated by focus ion beam, and the mechanical property evaluation was conducted by micro-compression tests.By controlling the electroplating parameters, a wide Cu content ranging from 5–25 wt.% was attained in the Au–Cu alloy films. Grain sizes of the Au–Cu alloy films estimated from X-ray diffraction measurements and Scherrer equation were 5–9 nm, which is advantageous in grain boundary hardening mechanism. When the Cu content was controlled at 14 wt.%, an ultrahigh yield strength at 1.35 GPa was obtained, which is comparable to that of Si materials. The yield strength was higher than the values reported for Au-based materials in the literatures [4] and suggested to be a synergistic effect of the grain boundary hardening mechanism with the solid solution hardening mechanism. Moreover, a transition of deformation behavior from barrel to fracture deformation was observed when the Cu content exceeds 14 wt.%, which provides important information for the structural design of MEMS devices.Reference[1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett. 104 (2014) 074102.[2] T. Tsuchiya, O. Tabata, J. Sakata, Y. Taga, J. Microelectromech. Syst. 7 (1998) 106–113.[3] J.A. Sharon, Y. Zhang, F. Mompiou, M. Legros, K.J. Hemker, Scripta Mater. 75 (2014) 10–13.[4] H. Tang, C.-Y. Chen, T. Nagoshi, T.-F.M. Chang, D. Yamane, K Machida, K Masu, M. Sone, Electrochem. Commun. 72 (2016) 126–130.

It is important to achieve materials with large coefficient of thermal expansion in science and engineering applications. In this paper, we propose an experimentally-validated metamaterial approach to amplify the thermal expansion of materials based on the guiding principles of flexible hinges and displacement amplification mechanism. The thermal expansion property of the designed metamaterial is demonstrated by simulation and experiment with a temperature increase of 245K for the two-dimensional sample, and 475K for the three-dimensional sample. Both experimental and simulation results display amplified thermal expansion property of the metamaterial. The effective coefficient of thermal expansion of the metamaterials is demonstrated to be dependent on the size parameters of the structure, which means by appropriately tailoring these parameters, the thermal expansion of materials could be amplified with different amplification factor. This work provides an important method to control the thermal expansion coefficient of materials and could be applied in various industry domain.

8:00 PM - TC06.05.03

The Effect of Sample Size on Mechanical Properties of Nanocrystalline Nickel

The effect of sample size on the mechanical properties have been extensively studied in decades. However, there are only few works had been done for the nanocrystalline materials and the results were still controversial. For the deep understanding and the exploring potential usage of sample size effect in application while it can be used in the form of nanostructured films, investigation of sample size effect of nanocrystalline materials is very important.In the present study, nanocrystalline nickel (NCNi) was electrodeposited using emulsified electrolyte with supercritical CO2 with use of polyoxyethylene lauryl ether for formation of the emulsion. Temperature and pressure were kept at 327 K and 15 MPa for successful NCNi plating. Micro-pillars with square cross section ranging the size from 5 to 30 μm were fabricated using focused ion beam from the NCNi and purchased single crystal nickel (SCNi). Tapering which often affect the mechanical properties was avoided by fabrication method using ion irradiation from pillar side. Micro-compression tests were conducted with a custom made testing machine equipped with flat ended diamond tip. In micro-compression test of different sized samples, displacement rate was controlled to be a constant strain rate of 2.5 X10-3.Grain size of NCNi evaluated by measuring more than 500 grains in TEM image was 7.7 nm which comparable with 8.7 nm obtained from XRD. Crystal orientation of the SCNi pillar against loading axis fixed at <789> which is 5 degrees off from the <111> orientation. Large work hardening owing to the crystallographic geometry causing cross slip observed in compression followed by the softening. Corresponding multiple slip traces are observed on SCNi pillar surface after compression. On the other hand, broad shear band observed in NCNi pillars which crossing from top to bottom of the pillars. Such deformation behavior could be due to the grain boundary mediated deformation mechanisms operated while the dislocation storage in small grain diminished. The yield stress of NCNi pillar compression was more than 10 times larger than the peak stress of SCNi pillar before softening occurred. Both NCNi and SCNi indicated size dependent strength with scaling exponent, slope in log-log plot, of -0.25 for SCNi and -0.125 for NCNi. Although the exponent was small, strength extensively increased from 2.5 to 3.1 GPa when the sample size decreased from 30 to 5 μm. This strengthening can be explained assuming cooperative grain boundary sliding where the group of grains slides on the flat segment of grain boundaries. When sample size become smaller, small segments should be activated which need more stress than the activation of large segment.

Structural disorder can significantly affect the mechanical and physical-chemical properties of nano polycrystalline materials. However, the effect of interactions across different crystallites are still not well understood. The present study aims to clarify the effects of defect interactions on structural disorder over nano polycrystalline microstructures, exploiting the power of combining molecular-dynamics simulated microstructures with state-of-the-art line-profile analysis (LPA) of Debye-simulated X-ray powder diffraction profiles. Results from numerical simulations were also validated by comparison with analogous experimental observations.Structural alterations due to inter-crystallite interactions of local defects were predominant over intra-crystallite effects. Nonetheless, the parameters estimated via LPA for the polycrystalline system as a whole were in agreement with the expected ideal values, supporting the statistical reliability of state-of-the-art LPA methods. Expressed by the static component of the Debye-Waller parameter (BISO), the inter-crystallite structural disorder slightly increased with environmental temperature (T) and, conversely, significantly decreased as a function of increasing defect density (ρdefects). This may partially explain considerable variations in estimated Debye-Waller parameters reported in the literature for nano materials of the same composition. Finally, the dynamic disorder was also investigated across the crystallites, revealing anisotropic vibrational components of atoms at the grain boundaries.

Nanoporous gold(np-Au) have been widely studied for catalyst, sensor, actuator and other applications due to their high surface-to-volume ratio, bio-compatibility and chemical inertness. Np-Au shows brittle fracture by open cell structure that only few of ligament should bear tensile load. Brittleness of np-Au causes difficulty in use to MEMS devices and other applications, thereby mechanical properties of np-Au have been investigated. In recent studies, nanotwin(nt) structure in Cu and Ag can significantly enhance both strength and ductility compared to ultra-fine grain or coarse grain structure due to a large density of twin boundaries. In this study, we fabricated nanotwined nanoporous gold(nt np-Au) thin film and measured its mechanical properties using in-situ tensile test.We fabricated nt Ag-Au thin film by co-sputtering of Ag and Au targets in magnetron sputter. By changing annealing time after deposition, we controlled grain size and twin boundary density in nt precursor thin film. After free corrosion dealloying in nitric acid, microstructures of precursor and np-Au are observed by TEM(Transmission Electron Microscope) and SEM(Scanning Electron Microscope) and mechanical properties are investigated by in-situ tensile test using push-to-pull device in SEM chamber.

Nanoporous gold (np-Au) made by dealloying is composed of a bicontinuous network of ligaments (solid) and pores. This material has attracted attention in a variety of applications, such as catalysis, sensors, and actuators, due to its low weight and high specific surface area. Several studies of the mechanical properties of np-Au have shown that the Gibson-Ashby scaling equation for open foam materials cannot be applied directly to np-Au. Accurate scaling laws for np-Au are challenging to derive because of complex issues such as ligament size effect, tension-compression asymmetry, and geometric structure. The change in yield strength with ligament coarsening relies on ligament-size-dependent mechanical behavior (the smaller is the stronger) on the assumption that structures of np-Au are self-similar regardless of whether ligaments are coarsened. Few researchers have looked at the relationship between network structure and mechanical properties as well as structure of np-Au in terms of morphology, and topology. Thus, it is important to identify the structural change of np-Au as coarsening and effect of structure on mechanical properties. This study validates change in 3D structure of np-Au as coarsening and looks at the relationship between structure and mechanical behavior. We fabricated several np-Au samples with various ligament sizes from 60 nm to 1 um, using thermal coarsening at different temperatures and studied the 3D np-Au structures by FIB tomography so as to look at whether or not structure of np-Au is self-similar during structure coarsening. We analyzed the distribution of ligament size, surface-to-volume ratio, and scaled connectivity density for coarsened np-Au, revealing that np-Au coarsens in a self-similar way.

Signature parameters such as true activation volume and effective stress are often characterized to identify the governing plastic deformation mechanisms, including that of nanocrystalline metals. So far, these parameters have been mainly measured on macroscopic specimens. Here we demonstrate the use of a MEMS device to measure true activation volume and effective stress on small scale specimens, based on repeated stress relaxation and stress dip experiments, respectively. The technique was demonstrated on 100-nm-thick nanocrystalline Au microbeams (with micrometer lateral dimensions), as well as 200-nm-thick nanocrystalline and ultrafine grained Al microbeams. The true activation volume of the Au specimens was calculated to be around 5 b3 and the effective stress was about 130 MPa. The signal-to-noise ratio for these transient tests was found to be very good, proving the robustness of the technique. The mechanical stability of the device during these transient tests was also characterized using in-situ SEM tests. These miniaturized tests open up the possibility of observing the mechanisms directly under a transmission electron microscope and providing a direct link between these measured parameters and the governing mechanisms.

A cellular material consists of a lattice structure invested with vacancies or gaseous pores. Cellular materials come in different forms, from standard foams to highly ordered honeycomb structures, and can be created from a wide variety of bulk materials. Cellular materials usually take one of two forms, namely, ordered or disordered. An ordered foam has a unit cell that continuously repeats along all three coordinate planes creating a uniform structure. These foams tend to have material properties that are consistent all over their volume. A disordered foam has instead a heterogeneous cell structure that cannot be represented by a repeating unit cell. The varied cell structures of these disordered foams do not allow for consistent material properties and hence a disordered foam is typically rated for its properties at its weakest point. To better understand the relation between cell structure and mechanical properties, a series of compression simulations were performed on selected ordered foam unit cells. Various pore geometries (circular, triangular and square) were investigated using finite element method (FEM). Initial 3D computer-aided design (CAD) models of thin-walled ordered foams with suitable dimensions were created based on experimental foams and subsequently used for uniaxial compression FEM simulations. Elastic and plastic deformation behavior was studied to identify optimal foam cell geometry as related to material properties at specific relative densities. Simulation results in this study indicate the pore geometry has a direct effect on the mechanical behavior of the foams. The square pore geometry leads to the highest stiffness foams at a relative density level of 11% and higher. In addition, the triangular pore geometry foams have a greater stiffness at a relative density of lower than 11%. This research contributes to improving understanding of foams and cellular materials and enhancing design and fabrication of human-made cellular materials with affordable additive manufacturing methods.

Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, although the origin of this high elastic modulus is unclear. Here, we investigate the structural features that govern mechanical behavior in ordered nanocrystal solids, or superlattices, by using polymer-grafted nanocrystals to create superlattices with tunable architecture. Colloidal self-assembly is used to arrange polystyrene-grafted Au nanocrystals at a fluid interface to form nanocrystal superlattices with sub-10 nm features but overall dimensions of ~1 cm. Thin-film buckling is used to determine the elastic modulus of the nanocrystal superlattices, which is found to be ∼6–19 GPa for superlattices containing 3–20 vol % Au. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be the critical parameters that govern superlattice elasticity, rather than the spatial arrangement of nanocrystals. Nanoindentation is used to probe plastic deformation in superlattice thick films. Hardness is found to increase with increasing Au volume fraction, and to range from 120-170 MPa.

8:00 PM - TC06.05.10

The Nanomechanics and Size Effects on Mechanical Properties of Nanocrystalline Aluminum

Aluminum, one of the most widely used lightweight metals, and its alloys are materials with promising continuing applications to meet the future challenge of pollution reduction and energy efficient transportations. There is an urgent need to design and develop ultra-strong Al based alloys. Nanostructuring is considered as one of the most efficient ways to improve the mechanical properties of material systems. In order to design nanocrystalline metals and alloys with optimum and/or tunable mechanical properties it is important to quantify their grain size dependences. Based on reliable interatomic potential that has been repeatedly verified by various experiments, we carried out comprehensive classical molecular dynamics (MD) simulations of tensile deformation for nanocrystalline Al samples with a broad range of different mean grain sizes. The largest nanocrystalline Al sample has a mean grain size of about 30 nm and contains over 100 millions atoms in the modeling system. The grain size dependences of elastic modulus, ultimate tensile strength, yield strength, and atomic fraction of dislocations are quantified and the size dependence mechanisms are also revealed. Particularly, the complete Hall-Petch relationship for the nanocrystalline Al bulk is determined, from which three distinct regions are identified including the normal, inverse, and extended regions in the Hall-Petch relationship. We expect these findings will provide useful insights in the nanomechanics of nanocrystalline Al. The quantitation analyses on dislocations and mechanical properties may facilitate the design and development of high strength nanocrystalline Al and Al based alloys as well as future testing procedures for promising structural and transportation applications.

Nanoporous Gold (NPG) made by dealloying takes the form of macroscopic millimetre- to centimetre-sized porous bodies with a solid fraction around 30%. The material exhibits a uniform, bicontinuous network of nanoscale pores and solid ligaments. The relationship between the solid fraction and the macroscopic mechanical behavior is commonly described by the Gibson-Ashby scaling laws for open cell foams. However, one main observation of various experimental studies is a much lower leading constant, in several cases combined with a higher exponent within the Gibson-Ashby scaling law for the Young’s modulus.For deeper understanding of the mechanical behavior and the occurring deformation mechanisms, various modelling approaches are suggested in literature, which simplify the complex NPG network to unit cell structures such as cubic, diamond or three point bending beam. This significant simplification and yet lack of satisfactory understanding of the underlying structure-property relationship brings the risk of losing detailed insight into the real NPG structure, and thus, possibly important aspects of its mechanical behavior.Liu et al. (2016) attribute the discrepancy between experiments and the Gibson-Ashby scaling law to a lowered network connectivity of the NPG structure seen in SEM-images: Some ligaments are dangling ligaments and thus cannot carry external load. They introduced the “effective relative density” which is much lower than the real relative density of the examined NPG samples. In the study of Hu et al. (2016) tomographic reconstructions of NPG samples using a dual-beam FIB and SEM were carried out for the first time. For different samples with varying ligament size, it was reported that the effective solid fraction is approximately half of the measured solid fraction. The authors likewise show, that the effective load-bearing ring structure governs the mechanical behavior, rather than the solid volume fraction.This paper presents a computational cheap FEM beam model, which is based on skeletonization of such 3D FIB-SEM tomography data of NPG to bridge computer simulations and experiments. The FEM skeleton beam model is validated using a FEM solid model generated from the very same 3D tomographic data. Various FEM skeleton beam models are generated for varying sample sizes out of the 3D FIB-SEM tomography. The resulting macroscopic properties are analysed in dependence of structural characteristic parameters, such as effective solid fraction and connectivity, and are compared to results obtained from idealized diamond unit cell structures. The models allow clarifying the role of dangling ligaments and load bearing rings in terms of the effective relative density. Furthermore, concerning the diamond structure, the FEM skeleton beam model provides an answer to the question of the percentage of nodes that are connected with three respectively four ligaments, needed for predicting a macroscopic response close to the real NPG.

Gold thin films are of technological importance in many applications, especially electronics and micro electromechanical systems, where their thermo-mechanical response is critical. Additionally, gold is of interest in high-pressure and dynamic research for use as a standard because of its high compressibility and inert character, and in understanding earth science and astrophysical processes. Characterizing the relationships between film thickness, grain size, and grain aspect ratio together is important in resolving the relative influences of scale, bulk material properties, and defect contributions to properties. We present a study of electroplated films of gold with tailored microstructures up to thicknesses of several mm, in which we relate the grain structure of the film to mechanical response. Constant potential and pulsed plating techniques were used to create a variety of grain structures, including high aspect ratio columnar grains. In situ scanned probe microscopy and ex situ microscopy and electron back-scattering diffraction characterization of the film deposition process and grain structure was correlated with micro- and nano-mechanical characterization of the films.

The mechanical property of metallic nano-foams is of great interest, as it opens new possibility of engineering materials with a tailorable specific strength. While nano-foams have been proposed for a range of functional applications (invoking unique chemical or optical properties), the mechanical durability of the nano-foam is a crucial aspect of realizing many applications. While de-alloying allows exquisite control of structure, this method is not conducive to forming relative densities on the order of 1-10% of the bulk material. To explore the low density regime of metallic nano-foams, Cu nano-foams were synthesized using electrospun non-woven fibers of polymer-metal precursor mixtures as template. The polymeric structures were then pyrolyzed and the resulting oxide structures reduced in a hydrogen environment to form polycrystalline foams with ligaments on the order of 50-250 nm in diameter with low relative densities. The mechanical properties (strength and modulus) were evaluated by nanoindentation using a flat-punch tip. The morphology of nano-foam was determined by scanning electron microscopy (SEM). The polycrystalline structure of nano-foam was identified and quantified using high-resolution transmission electron microscopy (HR-TEM). A phase grating multi-slice approach by Barthel and Houben was used to simulate HR-TEM to obtain proper interpretation of the experimental images, with sub-10 nm grains forming the ligaments. The experimental observations of strength, deformation mechanisms, and relative structural features (both at the grain level and at the larger architectural level of ligament size and spacing) offer a useful insight on several key aspects to modeling in computer simulations at the atomistic scale. Observations of the deformation mechanism of the nano-foam are crucial to develop physically meaningful model structures and construct boundary conditions for multiscale modeling, in particular focusing on the deformation balance between ligaments in tension, torsion, and bending. We discuss these in detail based on our experimental observations.

Architected cellular solids are a fundamental part of nature’s repertoire of structural materials. Just a few examples include coral, wood, and cancellous bone. These biological materials each contain a microstructure that has been customized to suit the requirements of the parent organism. With the advent of micro- and nanoscale 3D printing, we can now similarly program the architecture of synthetic cellular materials to achieve specific mechanical properties. There is great potential for this class of materials for purposes such as lightweight structural support and energy absorption in aerospace and automotive structures.

The mechanical properties of cellular materials with a single solid phase are generally described by scaling laws which are dependent on the relative density of the material. This paper extends these scaling laws to encompass composite cellular materials with multiple solid phases. We also outline a simple process for the fabrication of polymer-metal composite lattices at the mesoscale, with unit cells on the order of ~100 microns. Polymer lattice templates are fabricated with high resolution by two-photon lithography and then metallized by electroless plating to create composite cross sections. We observe the mechanical behavior of these composite lattices under uniaxial compressive loading, and we compare the experimental results to theoretical predictions from the generalized scaling laws.

Preliminary results show the strengthening effects of additional metallic layers deposited around the original polymeric cores of the lattices. An increase in the thickness of the metal shell correlates with an increase in the stiffness and strength of the overall lattice material. These results demonstrate that with modern fabrication techniques, we could precisely tailor the next generation of cellular materials for a variety of applications.

Graphene-based nanomaterials have exceptional electronic, mechanical, and thermal properties that can be tuned by precise control of their nanostructural features. Such tunable properties are responsible for the unique function of these nanomaterials that has potential to enable numerous technological applications. Toward this end, in this presentation, we report a systematic computational analysis of the mechanical behavior of a class of two-dimensional (2D) carbon-based nanostructures, namely, graphene-diamond nanocomposites formed through interlayer covalent bonding of twisted bilayer graphene with commensurate bilayers. The interlayer bonding is induced by patterned hydrogenation that leads to formation of superlattices of 2D nanodiamond domains embedded between the two graphene layers and having the periodicity of the underlying Moiré pattern. The analysis is based on molecular-dynamics simulations of uniaxial tensile straining tests according to a reliable interatomic bond-order potential. The mechanical response of these carbon nanocomposite structures is explored systematically as a function of their structural parameters, which include the commensurate bilayer’s twist angle, the stacking type of the nanodomains where the interlayer bonds are formed, the interlayer bond pattern and density, and the concentration of sp3-bonded C atoms in these superstructures. We determine the mechanical properties of these 2D materials and identify a range of structural parameters over which their fracture is ductile, mediated by void formation, growth, and coalescence, in contrast to the typical brittle fracture of graphene. We introduce a ductility metric, demonstrate its direct dependence on the concentration of sp3-bonded C atoms, and show that increasing the concentration of sp3-bonded C atoms beyond a critical level induces ductile mechanical response. We analyze the ductile fracture mechanisms and probe the brittle-to-ductile transition. Our study sets the stage for designing few-layer-graphene-based nanocomposites with unique mechanical properties and functionality.

We performed atomistic molecular dynamic simulations to study the mechanical properties of combinations of multi-layered heterostructures of 2D nanomaterials, including graphene and hexagonal boron nitride (hBN). We chose different stacking order of the free-standing 2D nanofilms and performed simulated indentation analogous to indentation via atomic force microscopy (AFM). Elastic properties and intrinsic breaking strengths including Young's modulus, bending modulus, ultimate tensile strength, and fracture strain were measured through nanoindentation simulation and compared to monolayer and bilayer 2D nanostructures. Our results suggested the heterostructures were comparatively more robust than their mono- and bi-layer counterparts. However, the indented area of hBN was much smoother than graphene which had comparatively rough fractured area. The magnitude of strength and fracture strain of monolayer and bilayer graphene were marginally greater than their respective hBN layers whereas that of three layered heterostructures were substantially stronger than their counterpart homostructures.

8:00 PM - TC06.05.18

Modeling and Measurements of XRD Spectra of Extended Solids under High Pressure

We present results of evolutionary simulations based on density functional calculations of various extended solids: N-Si, N-P, and N-H using variable and fixed concentration methods of USPEX. Predicted from the evolutionary simulations, high density structures with covalent bonds were analyzed in terms of thermo-dynamical stability and agreement with experimental X-ray diffraction spectra. Stability of the predicted system was estimated from convex-hull plots. X-ray diffraction spectra were calculated using a virtual diffraction algorithm which computes kinematic diffraction intensity in three-dimensional reciprocal space before being reduced to a two-theta line profile. Direct computation of the structure factor enables the use of distinct atomic scattering factors for each atomic constituent, thus allowing calculation of diffraction patterns for multicomponent systems. Calculations of the XRD spectra were used to search for a structure of extended solids at certain pressures with best fits to experimental data according to experimental XRD peak position, peak intensity and theoretically calculated enthalpy. Comparison of Raman and IR spectra calculated for best fitted structures with available experimental data shows reasonable agreement for certain vibration modes.

Part of this work was performed by LLNL under Contract DE-AC52-07NA27344. We thank the Joint DoD / DOE Munitions Technology Development Program and the HE C-II research program at LLNL for supporting this study. Portions of this work were performed at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

8:00 PM - TC06.05.20

The Mechanics of Failure of Grain Boundaries in Graphene Growth via Chemical Vapor Deposition—Simulations and Experiments

Graphene consists of a single layer of carbon atoms bonded covalently in a hexagonal lattice. It is the strongest material ever characterized. The elastic modulus and breaking strength were determined experimentally through the nanoindentation of freely-suspended circular graphene membranes. In order to take full advantage of these mechanical properties, industrially scalable synthesis methods are required. Chemical vapor deposition (CVD) has proven a fruitful method for growing large area continuous monolayers of graphene on top of copper substrates. While the growth method introduces grain boundaries, folds, and wrinkles into the monolayer film, subsequent nanoindentation experiments have shown that these defects only slightly diminish the mechanical strength. Herein, we present methods for improving the quality of CVD-grown graphene with oxygen-assisted methods that eliminates the substrate dependency on the tunability of graphene’s grain size. We also present electro-polishing techniques that yields few nanometer surface roughness over large areas, therefore resulting in extremely flat graphene. The application of these techniques leads to a higher quality film and therefore superior mechanical performance. Additionally, we present the numerical formulation for a membrane-based cohesive zone model to study the onset of failure in grain boundaries of polycrystalline graphene. This model is implemented within the context of the finite element method to study the failure mechanism based on grain boundary proximity and morphology in relation to the indentation tip.

8:00 PM - TC06.05.21

Resilience of Pristine Graphene to Large Thermal Fluctuations in Graphene-Titanium Interfaces

Physical and chemical properties of graphene-metal interfaces have been largely studied in literature in view of the development of nanostructured carbon-based electronic nanodevices. Although electronic properties are key to these devices, structural, thermal and mechanical properties are important as well. One of the most studied is the graphene-titanium (G-Ti) interface. Titanium is a low density, high strength versatile metal that can form alloys for applications ranging from aerospace to medical. Small clusters and thin films of titanium deposited on graphene have been studied for different applications. However, while some experiments show that thin films of titanium on graphene can be removed without damaging graphene hexagonal structure, others reported the formation of titanium-carbide at G-Ti interfaces. Here, the resilience of pristine G-Ti interfaces to large thermal fluctuations is investigated using methods of classical molecular dynamics (MD). The Charge Optimized Many Body (COMB3) potential, recently parameterized to simulate Ti-C-O-H multicomponent systems, is used to simulate these structures. MD simulations at different temperatures of G-Ti structures laying on a cooper substrate show that G-Ti interfaces remain thermally stable. In order to verify the integrity of G-Ti interfaces under more reactive conditions, MD simulations of G-Ti on substrates with curved, kinked and suspended shapes are also considered. As COMB3 was parameterized to reproduce the formation of titanium-carbide, its absence in these MD simulations shows the resilience of pristine graphene in G-Ti interfaces and can help explain the various experimental results mentioned above.

In spite of years of intense research, graphene continues to produce surprising results. Recently [1], it was experimentally observed that under certain conditions graphene can self-drive its tearing and peeling from substrates. This process can generate long, micrometer sized, folded nanoribbons without the action of any external forces. Also, during this cracking-like propagation process, the width of the graphene folded ribbon continuously decreases and the process only stops when the width reaches about few hundreds nanometers in size. It is believed that interplay between the strain energy of folded regions, breaking of carbon-carbon covalent bonds, and adhesion of graphene-graphene and graphene-substrate are the most fundamental features of this process, although the detailed mechanisms at atomic scale remain unclear. In order to gain further insights on these processes we carried out fully atomistic reactive molecular dynamics simulations using the AIREBO potential as available in the LAMMPS [2] computational package. Although the reported tearing/peeling experimental observations were only to micrometer-sized structures, our results showed that they could also occur at nanometer scale. Our preliminary results suggest that the graphene tearing/peeling process originates from thermal energy fluctuations that results in broken bonds, followed by strain release that creates a local elastic wave that can either reinforce the process, similar to a whip cracking propagation [3], or undermine it by producing carbon dangling bonds that evolve to the formation of bonds between the two layers of graphene. As the process continues in time and the folded graphene decreases in width, the carbon-carbon bonds at the ribbon edge and interlayer bonds get less stressed, thermal fluctuations become unable to break them and the process stops [4].[1] J. Annett and G. L. W. Cross, Nature 535, 271 (2016).[2] http://lammps.sandia.gov[3] A. Goriely and T. McMillen, Phys. Rev. Lett. 88, 244301 (2002).[4] A. F. Fonseca and D. S. Galvao – submitted.

8:00 PM - TC06.05.23

Strain and Bond Length Dynamics upon Growth and Transfer of Graphene by Nexafs Spectroscopy from First Principles and Experiment

Post-CMOS technologies need to overcome the barriers of industrial-scale fabrication of large-area, defect-free graphene. Indeed, in order to fully exploit novel physical properties in graphene, transfer procedures onto desired substrates must be achieved under controlled conditions.Challenges to enhance CVD grown-transfer graphene include the minimization of in plane and out of plane defects resulting from processing that induce random strain fluctuations; which have been identified as the dominant disorder source in graphene devices. Indeed, strain and rippling effects, both generated early on as growth-derived prior to any processing mainly as consequence of lattice mismatch between metal substrates and graphene, as well as transfer-derived mechanical effects and impurities - adsorbed atoms or molecules from transfer mechanisms.

Recent studies highlighted the value of Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. This is a synchrotron technique based on the excitation of a core electron to an unoccupied antibonding state. Decay of the excited core electron yields information on molecular orientation and chemistry, suitable for the study of corrugation in graphene.

In this work, we employ advanced NEXAFS spectroscopy combined with high level simulations to predict lattice parameters of graphene grown on copper and further transferred to a variety of substrates. In this scheme, with the aid of theoretical standards calculated through first-principles, lattice constants of transferred graphene are predicted by virtue of previously-reported relationship between bond lengths and σ* shifts in NEXAFS spectroscopy. The strains associated with the predicted lattice parameters are in agreement with experimental findings. The approach presented here holds promise to effectively measure strain in graphene and other 2D systems at wafer levels to inform manufacturing environments.

Recently, Shunhong Zhang et al1 proposed a new class of carbon-based nanostructures, named ‘penta-graphene’. In this work, we report preliminary results for impact molecular dynamics for a monolayer of penta-graphene. It was investigated the mechanical response of these structures under ‘ballistic’ impact of diamond particle (entirely atomistic).The simulations were carried out using the LAMMPS2, which is a very suitable and flexible package for high quality molecular dynamics simulations. The force field employed was ReaxFF force field 3. The choice of incident particle as a diamond fragment was because this material has a well-known large mechanical resistance.We simulated impact dynamics of diamond particle formed of 68,372 carbon atoms with an initial kinect energy of 2.45x106 Kcal/mol, and a penta-graphene sheet formed of 177,504 carbon atoms totalizing 245,876 atoms. The sheet was first thermalized in a NPT ensemble keeping null pressure along x, and y directions. After, we performed a second thermalization in a NVT ensemble at 300K for the whole system, and finally the diamond ball was launched with an initial speed of 5 Km/s against the sheet in a NVE ensemble. The time step integration used was 0.02 fs with ReaxFF parameters of Muller et al4. After four picoseconds of dynamics, it occurs a fracture on the penta-graphene monolayer. The final kinect energy of diamond ball was 2.1x106 Kcal/mol and there was an absorption of almost 14% of kinect energy during the collision.The results presented here show that penta-graphene monolayer is very resistant under impact, and the speed for which the fracture occurs is around 5 Km/s.1 Zhang, Shunhong; Zhou, Jian; Wang, Qian; et al., Penta-graphene: A new carbon allotrope, PNAS, 112, 2372 (2015).2 S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, J. Comp Phys, 117, 1 (1995).3 A.C.T. van Duin, S. Dasgupta, F. Lorant, W.A. Goddard, ReaxFF: A reactive force field for hydrocarbons, Journal of Physical Chemistry A, 105, 9396 (2001).4J.E. Mueller, A.C.T. van Duin, and W.A. Goddard III Development and Validation of ReaxFF Reactive Force Field for Hydrocarbon Chemistry Catalyzed by Nickel J. Phys. Chem. C,114, 4939 (2010).

8:00 PM - TC06.05.25

Understanding Interfacial Mechanical Properties of Graphene Based Surfaces as a Function of Solvent and Substrate

Graphene and graphene oxide (GO) are drawing the interest of researchers due to the material’s tailorability and superior stiffness and strength compared to other paper-like materials. However, little is known about how the surface chemistry of GO affects the interfacial mechanical properties of graphene supported on SAMs, which have been widely used to engineer the electronic properties of substrate-supported graphene devices, or as a function of solvent. This study is a combination of contact resonance atomic force microscopy experiments coupled with all-atom and steered molecular dynamic simulations, which were used to show the affects of head group chemistry of SAMs on the out-of-plane elastic modulus of the graphene-SAM heterostructures. This collaborative study revealed graphene supported on hydrophobic SAMs were stiffer than those of graphene supported on hydrophilic SAMs, which was largely due to fewer water molecules present at the graphene-SAM interface with hydrophobic SAMs. The hydrophilic chain’s amino head group allowed for hydrogen bonding between the chain’s head groups. This disrupted the packing order of the SAMs by inducing a repulsion of the carbon chains and ultimately contributed to the decreased stiffness of the device with the hydrophilic chains. Moreover, we focus on the role of a solvent’s hydrophobicity on the mechanical properties of graphene-based surfaces, which are also a crucial factor for their mechanical stability. In this study, we chose to compare methanol, ethanol, and water due to graphene’s various dispersion behaviors in these solvents. Methanol is very limited in its ability to disperse graphene, ethanol disperses graphene with short-term stability, and water disperses graphene with long-term stability. The results from our simulations demonstrate that a solvent’s hydrophobicity can strongly impact the interfacial mechanical properties of graphene-based materials. These results provide an important, and often overlooked, insight into the mechanical properties of substrate-supported graphene surfaces.

Harnessing the unique properties of 2D materials and heterostructures for nanoscale device applications requires precise control over the van der Waals epitaxy, and the chemical and structural nature of the interface. The interface to the growth substrate, the layer number, the lateral arrangement of different domains, and the nature of subsurface structure elements, are all important variables to control. This calls for new characterization approaches to investigate subsurface features. We demonstrate that contact resonance atomic force microscopy (CR-AFM) can yield a quantitative, subsurface-structure sensitive, mechanical fingerprint of 2D layered materials. To deconvolute the experimentally measured, aggregate contact stiffness and to quantify the nano-mechanical stiffness contributions from each material layer, we developed a new method that combines ab initio and continuum modeling approaches to predict the aggregate contact stiffness. Furthermore, we report on the piezoelectric contrast, measured with piezoresponse force microscopy (PFM), between regions with different graphene layer numbers on 6H-SiC (0001). Through experiments on oxygen-intercalated, freestanding graphene and on tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) doped graphene on SiC, we show that the observed piezoelectric contrast arises from the interfacial dipole moment across the van der Waals (vdW)-bonded graphene and the partially σ-bonded zerolayer graphene, and is due to the spontaneous polarization of the bulk SiC substrate, which breaks the crystal symmetry in the out-of-plane direction. Our findings thus demonstrate a new way to turn graphene into a piezoelectric material by manipulating the graphene-substrate interface. Finally we discuss how the surface chemistry of self-assembled monolayers (SAMs) affects the interfacial mechanical properties of graphene on SAMs. Using CR-AFM we show that changes in interfacial mechanical properties can be characterized through out-of-plane elastic properties measurements. These measurements suggest that the observed differences are due to different amounts of water molecules associated with the different SAM head groups, present at the graphene-SAM interface. The experimentally observed stiffness differences are successfully captured via steered and all-atom Molecular Dynamics (MD) simulations.

Schwarzites are crystalline, 3D porous structures with stable negative curvature formed of sp2-hybridized carbon atoms [1]. These structures present topologies with tunable porous size and shape with unusual mechanical properties [2]. In this work, we have investigated the mechanical behavior under compressive/tensile strains and energy absorption of four different Schwarzites, through reactive molecular dynamics simulations, using the ReaxFF force field as available in the LAMMPS [3] code. We considered two Schwarzites families, the so-called gyroid and primitive [4] and two structures from each family. Our results [5] show that under mechanical compression these structures can be reduced to half of their original size before structural failure (fracture) occurs. Our results also show remarkable resilience under mechanical compression and ballistic impacts. We will also discuss recent efforts to synthesize them.

Hybrid nanoparticles which have inorganic cores and organic shells in brush like conformation present interesting properties, such as ability to form stable dispersions in solvents compatible to the organic shell’s structure. In adequate conditions, the nanoparticles may interact strongly with each other and the polymer matrix in which they are dispersed, constructing an intertwined network which has good mechanical stability.This work will present results for highly concentrated nanocomposites of polyurethane adhesive and magnetite nanoparticles functionalized with organic shell. The magnetite inorganic core’s diameter ranges from 5 to 15 nm and it represents 60% of the nanoparticles total mass.Nanocomposite were prepared by physical mixing of 50%wt up to 95%wt of nanoparticles in a commercial PU adhesive and then the solvent was allowed to evaporate. SEM and TEM analysis show that the composite films, although highly concentrated in nanoparticles, were very homogenous. This could only be possible due to the nanoparticles’ organic shell in a brush configuration and its compatibility with the matrix.Tensile properties were measured, showing the relationship between mass percentage of nanoparticles in the composite and elastic modulus. The results showed changes in the stress vs strain curve behavior as nanoparticles are added. This happens because the nanoparticles organic shell’s deformation behavior has greater influence in the nanocomposite’s elastic and plastic deformation regimes at such high concentrations. The mechanical behavior observed is not predicted by current composites modeling theories.Acknowledgments:FAPESP, CNPQ, CDMF-LIEC

Molecular Dynamics simulations of nanoindentation on Palladium Hydrogen (PdH) systems are performed using a spherical indenter. The effect of coherent twin boundary (CTB) on dislocation nucleation and propagation mechanisms have been investigated by studying the force-displacement curves with the dislocation morphologies. The results support the well-established hydrogen enhanced local plasticity (HELP) theory. Hydrogen aggregations at the CTB has created unique microstructure and dislocation morphologies. While the Young’s modulus and hardness of the twin boundary region of the PdH sample have shown obvious softening effect, the dislocation statistics indicated significantly lower dislocation density for this sample. The mechanism of dislocation growth and entanglements are briefly explained based on the dislocation morphologies.

8:00 PM - TC06.05.30

First Principles Prediction of Non-Schmid Behavior of Polycrystalline Mg

The non-Schmid behavior (NSB) in {10-12} deformation twinning (DT) of Mg is reported in recent experiments in polycrystalline Mg; the occurrence frequency of {10-12} DT is considerably deviated from the Schmid’s law [1]. Why the NSB appears in {10-12} DT, but not in other DTs is still unclear. In this study, to reveal the NSB origin, we performed first principles shear deformation tests on the DTs of Mg crystal. We found that the normal strains other than twinning shear strain appear during the shear deformation, and the experimental occurrence frequency of the {10-12} DT of the polycrystalline Mg is successfully reproduced by considering the normal strains. This strongly suggests the normal strains are the NSB origin.

Micro- and nano-sized structures have attracted substantial interest due to their special mechanical behaviour: they generally show an increased yield strength compared with the bulk material as well as an improved ductility. Among them, nanoparticles (NPs) which are generally used for their shape-dependent functional properties also appear as perfect candidates for submicronic plasticity investigations due to their broad range of sizes and their variety of shapes. While metallic and silicon NPs have been widely investigated since almost two decades, less is known about the strength of ceramic NPs maybe due to the brittleness of their bulk counterparts.

In the light of recent but preliminary in situ TEM observations, we investigate here the mechanical behaviour of <100>-oriented MgO nanocubes. First, incipient plasticity mechanisms are investigated using molecular dynamics simulations (MD) of virtual nanocompression tests at constant strain rate. Results show that the plastic deformation of MgO nanocubes starts with the nucleation from the surface of perfect 1/2<110>{110} dislocations under ultra-high stresses, one order of magnitude larger than what is generally observed in fcc metals. However, MD calculations can only be carried out at strain rates several order of magnitude larger than experimental strain rates. MD calculation therefore do not give enough time to thermally activated nucleation to occur at lower stress and the nucleation stress is overestimated.By using the nudged-elastic-band method, we calculate the activation energy for the nucleation of the dislocation as a function of the stress, particle size and dislocation nucleation site. With the help of transition state theory, the nucleation stress can then be estimated as a function of temperature and experimental strain rate. We find that due to the large stiffness of the activation energy vs stress curve, the strain rate has a much smaller influence on the nucleation stress than what is found for metallic NPs and therefore, that the nucleation stress estimated from MD is close to that corresponding to experimental strain rate.

8:00 PM - TC06.05.32

Plastic Deformation of Sub-Micron Al and Be Wires—A TEM and In Situ TEM Study

The origin of the improved strength of sub-micron single crystals and whiskers is still debated, but after studies concentrated solely on size effects, it appeared that an as important parameter was the dislocation content of these small crystals.In this presentation, the importance of the dislocation content and the role played by the external surface on the triggering of plasticity in both Al and Be sub-micron wires investigated by in-situ transmission electron microscopy (TEM) will be highlighted. The wires, obtained by selective etching of Al/Al2Cu and Al/Be eutectic alloys (Fig.1), all exhibit a thin Al oxide outer layer. Al wires present a large variability in dislocation density while Be wires parallel to their c-axis are usually dislocation free or contain very few dislocations.In Al, we show that multiplication of dislocations through intermittent spiral sources directly causes a power-law increase of the yield stress with decreasing cross-sectional size. The size effect and resulting mechanical response are directly linked to the initial defect density and the distance between the source and the surface. In the absence of dislocations, fibers elastically reach high stresses with limited to no plasticity, reminiscent of whisker behavior.A similar fragile-like behavior is also observed in dislocation free Be wires. In this case moreover, the plastic deformation is strongly dependent on the orientation of the crystal with respect to the straining axis. When strained along their c axis, wires tend to twin. In twinned area, ductile behavior was observed due to the favorable orientation for prismatic slip. Twin nucleation and propagation is thought to be triggered by surface nucleation. Because of the presence of a remaining Al oxide surrounding the wire, we show that the deformation may require dislocations moving along the fiber axis. Our observations indicate that these dislocations are thought to move in or close to a remaining Al/Al oxide layer at the wire surface. In any case, no "special" mechanism is needed to explain the unusual properties of these wires.

The T91 ferritic/martensitic (F/M) steel is expected as a structural material for liquid lead-bismuth eutectic coolant reactors in Gen IV. However, the molten lead-bismuth eutectic alloy (mLBE) often causes the liquid metal embrittlement (LME) of the F/M steels. Although the prior austenite grain boundaries (PAGBs) and martensite block boundaries were reported to be preferential sites for the LME, the mechanism of the LBE of the F/M steels has not yet been comprehensively understood. We have also performed the mLBE corrosion tests for the T91 steel and found a reduction of hardness in the area near the surface contacted with the mLBE. However tensile tests for mLBE-corroded specimens did not necessarily show the LME. This is probably because the corroded area must have been limited within the area very near the surfaces. Accordingly, we fabricated micropillars in the area where the reduction of hardness was observed after the mLBE corrosion test and performed compression tests in order to understand the mechanism of LME of the T91 steel. The material used in this study was the 9Cr F/M steel T91. This steel was austenitized at 1343 K for 1 h and then air cooling, followed by a tempering heat treatment at 1043 K for 1 h. The liquid metal corrosion tests were performed with a notched plate (30 mm×15 mm×1 mm) after sensitization at 773 K for 20 min. The specimen was stressed by a CBB test jig and contacted with a LBE (44.8Pb-55.2Bi (wt%)), then heated at 773 K for 48 h in Ar gas flow. Micropillars with 3 μm in diameter and 12 μm in height were fabricated using a focused-ion beam (FIB). Micropillar compression tests were performed at a strain rate of 1×10-3 s-1 on the HYSITRON PI 85 SEM PicoIndenter equipped with a diamond flat punch. After compression test, the micropillars were observed by SEM and the microstructure was analyzed by SEM-EBSD, EDS and TEM. The stress-strain curves for the T91 revealed the yield stress to be 550-750 MPa for specimens prior to the mLBE corrosion test but to be about 200 MPa for those of post-mLBE corrosion test. Of particular interest was a finding that the micropillar made from the mLBE-corroded specimen was significantly sheared along a high angle boundary inclined at the angle of about 45° to the loading axis, whereas no brittle cracking was observed. In addition, EDS analysis detected the presence of Pb/Bi on this grain boundary. From the results obtained, it is likely that the mLBE penetrated into the grain boundaries and formed a thin second phase, which is probably responsible for the shear deformation along the grain boundary at a lower stress.

Fatigue is one of the most damaging mechanisms in structural components. With the development of structural nanomaterials, it is imperative to investigate the fatigue damage phenomena at the atomic scale. In this study, atomistic simulations of the fatigue behavior of polycrystalline nickel nanowire has been carried out to understand the basic mechanisms of fatigue damage under low-cycle fatigue loading. Shear strain tensor analysis is used to differentiate deformation mechanisms accommodating strain among nanowires. In addition, a computational analysis method is used to quantify plastic and elastic deformation. We combine our simulation results with the experimental results from quantitative in situ SEM tensile fatigue testing of pristine nickel nanowire. We found out pristine polycrystalline crystal Ni nanowires fail under fatigue loading in both simulation and experimental studies, which is a rare case for pristine nanowires without any cracks. Therefore, the collective effects from grain boundary, surface roughness as well as dislocations are playing a role in failure under cyclic loading. Further insights on the mechanisms contributing to the observed behavior are presented and discussed.

8:00 PM - TC06.05.35

Comprehensive Formation Mechanism of Annealing and Deformation Twinning in FeCrNiCoAlx and FeCrNiCoMox

It is commonly acknowledged that stacking faults, as defects of disordered crystallographic planes, are one of the most important slipping mechanisms in the most commonly seen lattice, face-centered cubic (FCC). Such defects can originate twinning formation resulting in the strengthen of mechanical properties, e.g. twinning-induced plasticity (TWIP), of high entropy alloys (HEAs) at cryogenic temperatures. In this work, by using density functional theory (DFT), the stacking fault originated twinning formation mechanism are discussed with regard to two different solute elements, aluminum and molybdenum, in the FeNiCoCr high entropy alloys. We also use phonon calculation combined with DFT calculation to thermodynamically determine the stability of FCC and HCP phase for these two HEAs. Our results show that adding aluminum has noticeable enhancement of twinning formation compared to molybdenum on the electron distribution and mean-square atomic displacement (MSAD) basis.

In the paper, the molecular dynamics (MD) method with the embedded atomic model (EAM) potential function was used to simulate the diffusion bonding process of Ti6-Al4-V alloy. The atomic model of Ti6-Al4-V was created by random substitution method in LAMMPS software. The relaxation and aging process of Ti6-Al4-V atom model under ideal conditions were simulated, and the phase transition behavior of Ti6-Al4-V in aging process was obtained. The simulation results could reveal the transformation of the α phase from the meta-stable β phase at the time of aging, and the atomic concentration distribution of each phase at equilibrium state was obtained. Based on the above model, the atoms diffusion process at the interface of two titanium alloy blocks was numerically simulated, and the influence of diffusion temperature on the diffusion layer thickness was studied. The present results indicated that the Ti atoms diffused at the highest ratios at interface layer, followed by Vanadium atoms, while the Al atoms diffused at the lowest ratios. The diffusion temperature had a great influence on the diffusion bonding of the Ti6-Al4-V interface. The higher the temperature is, the faster the diffusion rate is and the more disordered the atoms is at the interface. In addition, higher temperatures lead to larger diffusion layer thickness.

8:00 PM - TC06.05.38

The Effect of Size on the Resonant Frequencies of Metallic Nanowires in Molecular Dynamics Simulations

In the recent years, there has been a growing use of nanowires as basic building blocks in specimen and devices, such as in the nano-electro-mechanical systems (NEMS). Nanowires are characterized by their small dimensions, which can provide components with high force sensitivity, low mass and very high eigenfrequency spectra. In this study we examine how the size of nanowires affects their mechanical properties and the resonant frequencies of nanowires in molecular dynamics (MD) simulations. Au nanowires, both single crystalline and five-fold twinned, were deformed in MD simulations and then were allowed to vibrate both under oscillatory longitudinal and transversal modes. The resonant frequencies of nanowires of different lengths and diameters were calculated using the MD simulations and they were found to be size dependent. In addition, the elastic properties of the nanowires were found also to be size-dependent; Their Young's modulus decreases with the diameter, whereas the shear modulus increases. In order to interpret the results, a Timoshenko beam model was found to reproduce the MD simulation results. Using this model it is shown that the size-dependent mechanical properties manifest themselves in the longitudinal and transversal resonant frequencies of the nanowire. In addition, non-linear effects of energy transfer between oscillatory modes were identified. The MD-informed continuum model allows us to quantify the resonant frequencies of nanowires with microstructural characteristics without their atomic description.

Molecular crystals are composed from molecules arranged in a low symmetry crystal lattice. The structure is stabilized by weak van der Waals and long range electrostatic interactions. The intramolecular interactions are strong, while the intermolecular interactions are relatively weak. The nature and mobility of defects in molecular crystals is poorly understood. In this work, we investigate and characterize slip systems in β-HMX using computational tools. Experimental studies by Gallagher et al have suggested dislocation glide on (001) [100] and (101) [10-1] systems. Using the Smith-Bharadwaj potential calibrated for HMX, we estimate the Peierls stresses for dislocation motion for these slip systems, as well as other potential candidates.

The achievement of both strength and ductility is a significant demand for most engineered materials. Yet they are hard to reach coinstantaneous. Fortunately, natural materials have such solutions to deal with this dilemma. But can we apply the same strategy from natural system to engineering complex? We show here how the bio-inspired hierarchical steel was designed attaining unprecedented combinations of properties and how the synergetic effect cooperated on multiple length scale. The superior properties enabled by this strategy provide directions for all future strong material design requirement.Materials can be made strong, but as such they are often brittle and prone to fracture when under stress. Inspired by the exceptionally strong and ductile structure of byssal threads in the shells of mussels, we have designed and manufactured a multi-hierarchical steel which “defeats this “conflict” by possessing both superior strength and ductility. These excellent mechanical properties are realized by structurally introducing sandwich structures at both the macro- and nano-scale, the latter via an isometric, alternating, dual-phase crystal phase comprising nano-band austenite and nano-lamellar martensite, without change in chemical composition. Our experimental (transmission and scanning electron microscopy, electron back-scattered diffraction, nano-indentation and tensile tests) and constitutive simulation results revealed how this system works. This synergy is key to the development of vastly superior mechanical properties, and may provide a unique strategy for the future development of new super strong and tough (damage-tolerant) structural materials.

Hexagonal-close packed metals have diverse deformation modes of dislocation slip and deformation twining. Large differences in the activation energies between deformation modes at a specific loading condition cause a highly anisotropic deformation behavior. Moreover, if the sample size decreases below sub-micron scales, the deformation mode is subject to change to another one. In this study, we performed in-situ nanomechanical compression tests on submicron Mg pillars in transmission electron microscope as well as in scanning electron microscope to investigate the activation of specific deformation mode with a change of crystal orientation and its effect on deformation behavior and mechanical properties. Submicron pillars with sizes of 0.5 μm, 1 μm, 2 μm, and 4 μm were prepared along different orientations ([0001], [2-1-12], [10-10], [2-1-10] by using focused ion beam. The [0001]- and the [2-1-12]-oriented pillars are deformed predominantly by pyramidal slip and basal slip, respectively, which is easily accounted for by considering the Schmid factor and the critical resolved shear stress for each orientation. However, the mechanical size effect is quite different for the two dislocation slip processes, i.e., the size exponent of basal slip is two times larger than that of pyramidal slip. We found that for the basal slip process the exhaustion of mobile dislocations leads to strengthening of the pillars whereas for the pyramidal slip processes the accumulation of dislocations results in forest hardening with the decrease of pillar size. For [10-10] and [2-1-10]-oriented pillars, {10-12}<10-11> type tensile twinning is observed as a dominant deformation mode. However, after the nucleation of tensile twin the plastic flow is completely different for the two orientations. In the case of [10-10]-oriented pillars, the post-yield plastic flow is fully accommodated by the propagation of twin(s). In the case of [2-1-10]-oriented pillar, however, basal slip is easily activated within twinned region and dominate the plastic flow because the twinning-induced crystal reorientation changes the loading axis from the [2-1-10] to the [2-1-13] which favors the ease activation of basal slip within the twin.

8:45 AM - TC06.06.02

Proliferation of Twinning in HCP Metals—Application to Magnesium Alloys

Hexagonal close-packed (HCP) materials, such as magnesium alloys, are highly promising for the design of the next generation of lightweight, strong alloys. Because of the crystal structure, the mechanisms by which HCP materials accommodate deformation are not particularly well-understood. In particular, twinning - a symmetric reorientation of the material lattice about a planar discontinuity - has been a subject of contention, as there have been many observations of so-called "anomalous" modes that do not necessarily agree with classical theory. In order to address the existence of these anomalous modes and attempt to predict new modes in these materials, we develop a three-step framework for twinning. We begin by kinematically predicting all of the possible twin modes, after which we study the energetics of all of these modes and then estimate the stress necessary to observe these modes. Applying this in particular to magnesium, we show that there are a significant number of twin modes which actually participate in governing the yield behavior of magnesium; many of these modes were not previously predicted and some of these modes match experimentally-observed anomalies. We also discuss generalizations of this framework to additional materials - both HCP and non-HCP.

Metal nanocomposites present significant advantages over conventional bulk metals, such as high strength and damage tolerance, fatigue and radiation resistance, and high strength-to-weight ratio. However, an impediment to their widespread technological use is that they fail suddenly, with minimal uniform plastic deformation, due to flow localization. Here we show that flow localization can be averted by engineering the microstructure morphology in metal nanocomposites via solid and liquid phase dealloying. Using this technique we fabricated model materials with different microstructural length scales – from 30 nm to 100 nm – and different morphologies – globular, lamellar, and bicontinuous – and evaluated their mechanical behavior using nanoindentation. By examining material extrusion (i.e., pile-up) around the indents, we identified the effect of length scales and morphologies on flow localization. This work identifies microstructure morphologies that can avert flow localization, enabling large-scale, uniform, plastic deformation in metal nanocomposites, markedly advancing these materials’ potential for use in structural and functional applications.

9:15 AM - TC06.06.04

Mechanical Properties of Single Crystalline and Electrodeposited Lithium at Small Scales

Lithium metal is the most energy dense anode material, with a gravimetric energy density of 3860 mAh/g1. Lithium’s use in commercial batteries has been largely limited by the formation of dendrites during cycling in liquid and solid electrolytes. It has been postulated that a sufficiently strong and stiff solid electrolyte can mechanically suppress dendrite formation and growth2. Establishing the parameter space for stiffness and strength of lithium as a function of its microstructure and orientation is essential to enable dendrite suppression. Very few studies on the mechanical properties of lithium exist because of its high reactivity with moisture and air.

In our previous work, we performed nanomechanical experiments on single crystalline micropillars of solidified molten lithium using an in-situ nanomechanical module in a scanning electron microscope (SEM) under vacuum. We found that micron sized lithium has a strength of 105 MPa, 2 orders of magnitude higher than its bulk polycrystalline value. We also reported a high elastic modulus anisotropy for lithium, with a factor of 4 times between the modulus of the stiffest and most compliant crystallographic orientations3.

To investigate the mechanical properties of cycled lithium used in a real battery, we performed similar site- and size-specific nanomechanical experiments on lithium cycled in a commercial lithium metal secondary battery4 and on lithium electrodeposited using a liquid electrolyte. Ne+ ions were used to clean off any damage from Ga+ focused ion beam (FIB) milling during pillar fabrication. We discuss the mechanical properties of electrodeposited lithium cycled with these solid and liquid electrolytes as a function of sample size and microstructure. The results have implications for lithium dendrite suppression using robust solid electrolytes for the next generation of lithium metal secondary batteries.

Deformation twinning contributes to a high work-hardening rate through modification of the dislocation structure and a dynamic Hall-Petch effect in many polycrystalline metallic materials. Due to the well-defined compression axis and limited deformation volume of micro-pillars, micro-compression testing is a suitable method to investigate the mechanisms of deformation twinning and the interaction of dislocations with twin boundaries. The material investigated is an austenitic Fe-22wt%Mn-0.6wt%C twining-induced plasticity steel. Micro-pillars oriented preferentially for deformation twinning and dislocation glide are compressed and the activated deformation systems are characterized. We observe that characteristic dislocation and deformation twin boundary interactions induce unstable stress-strain responses in micro-pillars oriented for deformation twinning, while dislocation glide dominated deformation results in a more stable plastic flow behavior. The higher flow stresses and unstable stress-strain responses in micro-pillars oriented for deformation twinning are assumed to be caused by the activation of secondary slip systems and accumulated plastic deformation.

9:45 AM - TC06.06.06

Size Effect for Superelasticity in Shape Memory Alloys at the Nanoscale—Scaling Power-Law and Atomistic Model

Shape memory alloys are promising materials to be incorporated in new generation of smart micro electro-mechanical systems but, in order to guarantee a reproducible and reliable behaviour, a still open question must be answered: Whether the critical stress for the stress-induced martensitic transformation during superelasticity exhibits a size-effect similar to the one observed in confined plasticity. In the present work the answer to this crucial question is offered. A large series of [001] oriented pillars, from 2 mm down to 260 nm in diameter, have been milled by focused ion beam on Cu-Al-Ni shape memory alloy single crystals. The superelastic behaviour at the nano-scale has been carefully characterized by nano-compression tests and observed by in-situ test at the scanning electron microscope. Here we demonstrate that there is a remarkable size-effect on the critical stress for superelasticity. Even more, we have quantitatively determined, for the first time, the scaling power-law for superelasticity at small scale in shape memory alloys, and a model explaining the observed size-effect is proposed.

10:00 AM -

BREAK

TC06.07: Layered Structures

Session Chairs

Jon Molina-Aldareguia

Ruth Schwaiger

Tuesday AM, November 28, 2017

Hynes, Level 2, Room 210

10:30 AM - *TC06.07.01

Understanding the Role of Interfaces on Fully Lamellar TiAl Alloys through Micromechanical Testing

Fully lamellar gamma titanium aluminides are very promising materials for aerospace applications, due to their increased thrust-to-weight ratios and improved efficiency under aggressive environments at temperatures up to 750 °C. For that reason, they are projected to replace the heavier Ni- base superalloys currently used for low pressure turbine (LPT) blades manufacturing. However, their ductility is limited due to their inherent anisotropy, associated to their lamellar microstructure. The objective of this work was to study the mechanical response of single colonies of polycrystalline γ-TiAl, as a function of layer thickness and layer orientation, and to relate this mechanical response with the operative deformation mechanisms.With this aim, micropillars with lamellae oriented at 0°, 45° and 90° with respect to the loading direction were compressed at room and elevated temperature. A thorough study of pillar size effects revealed that the results were insensitive to pillar size for dimensions above 5 μm. The results can therefore be successfully applied for developing mesoscale plasticity models that capture the micromechanics of fully lamellar TiAl microstructures at larger length scales. The results revealed a large plastic anisotropy, that was rationalized, based on slip/twin trace analysis, according to the relative orientation of the main operative deformation modes with respect to the lamellar interfaces: longitudinal, mixed and transversal deformation modes. Additionally, microtensile specimens were also milled out of single colonies and in-situ tested in the SEM, to study the role of interlamellar interfaces on the plastic deformation and fracture under tension. EBSD was used before and after the test to study the role of different type of interfaces (true twin, pseudo twin and order variant) on slip/twin transfer.This study emphasizes the complexity of the micromechanics of fully lamellar TiAl alloys, where the activation of different deformation modes is strongly affected, not only by the lamellar orientation, but also by the character of the interfaces between the different lamellae.

Although the yield strength and elastic strain limit of submicron sized single crystals approach the theoretical limit, they exhibit minimal ductility compared to their bulk counterparts. Under tensile loading, dislocations in smaller volumes interact minimally with each other as they glide on remote and parallel slip planes before exiting from the sample surfaces. This diminishes their strain hardening ability and instead promotes strain localization along a shear offset. Enhancing the ductility of submicron sized single crystals is therefore a topic of significant technological and scientific interest. In this work, we used focused ion beam (FIB) to fabricate a unique submicron sized crystalline-glass bilayer configuration from a Zr based bulk metallic glass composite. The bilayer is loaded in tension inside a transmission electron microscope (TEM). We observed that the combination of a very soft bcc crystalline phase, bonded with a very thin layer of an amorphous phase, can sustain strains of up to 12.5%. In contrast, both the individual phases fail at a strain of ~4.5%. While dislocation slip initiates in the crystal at a stress concentration, on the (110) plane, at larger strains, geometrically necessary stacking faults are formed on the complementary (112) plane. Utilizing numerical simulations, we critically analyze the stress fields associated with the observed defects and assess the influence of individual components on the deformation response. The utility of this design strategy is further discussed in terms of creating failure resistant single crystals.

2-dimensional (2D) sharp interfaces with a distinct boundary demarcating an abrupt discontinuity in material properties in nanolayered composites have been studied for almost twenty years and are responsible for an increase in properties such as strength, radiation damage resistance, and deformability. However, 2-D interfaces have their limitations with respect to deformability and toughness. Moreover, nature frequently determines the interface character between two dissimilar materials, and oftentimes the resulting bimaterial interfaces are not chemically or structurally sharp. Thus, 3-dimensional (3D) interfaces may provide an entirely new opportunity to modify the interface structure, as well as the resulting properties. 3D interfaces are defined as heterophase interfaces that extend out of plane into the two crystals on either side and are chemically, crystallographically, and/or topologically divergent, in three dimensions, from both crystals they join. In this work, we study the structure/mechanical behavior correlation of Cu/Nb with chemically graded 3D interfaces as a model system to explore this idea. Micropillar compression results show that the strength of Cu/Nb nanocomposites containing 3D interfaces is significantly greater than those containing 2D interfaces. Shear banding in 3D Cu/Nb is observed during pillar compression with retention of continuous layers across the shear band. Microstructural characterization shows that large strain induces thinning of both the layers and 3D interface and eventually transforms the 3-D interface to a serrated morphology, which is thought to maintain favorable properties as has already been demonstrated in other nanolaminate systems. We will present our most recent results on microstructural evolution during deformation of such 3-D interfaces, and describe this evolution mechanistically through the use of atomistic simulations.

Multilayered metals of submicron layer thickness have desirable properties such as ultra-high strength, thermal stability, and radiation resistance. Yield strengths of these nanostructured materials are much higher than those of the constituent layers in bulk form. Effect of the layer thickness on strength is well studied; for layer thicknesses down to 100 nm, the strengthening is governed by Hall-Petch behavior. As layer thickness further decreases, dislocation pile-ups no longer form and rate of strengthening with decreasing layer thickness decreases. At layer thicknesses around 5 nm, a peak strength limited by the interface barrier strength is achieved. Research on nanocrystalline metals has shown that in addition to grain size refinement, alloying additions can also dramatically increase the strength. For example, 10% Nb addition to monolithic films of nanocrystalline Cu can improve the strength by a factor of 2.5. Although alloying additions are known to alter mechanical properties drastically, studies to date on multilayers have mostly focused on pure metallic components. In this work, we experimentally investigated the effect of alloying additions on the strength of multilayered composites. We selected Cu-Nb as the model system and prepared thin film multilayers made of alternating Cu90Ag10 and pure Nb layers with layer thicknesses in the range 10 nm – 100 nm. Cu90Nb10-Nb multilayers are also investigated for comparison.Thin films with a total thickness of 1 μm are prepared by magnetron sputtering on oxidized single crystal silicon, and characterized by X-ray diffraction and transmission electron microscopy. Layers are nanocrystalline with Ag and Nb uniformly distributed in the Cu matrix in solid solution form. Hardness of the films is measured by nanoindentation. Alloying additions to Cu layers dramatically increased the hardness of the multilayers when compared to pure Cu-pure Nb counterparts. Cu90Ag10-Nb and Cu90Nb10-Nb multilayers with 60 nm layer thickness have hardness of 6.0 and 6.5 GPa respectively, which is ≈50% higher than that of the multilayered Cu-Nb of same layer thickness. The achieved strength by alloying is close to the limit defined by the interface barrier of Cu-Nb to dislocation transmission. As layer thickness decreased, the enhancement obtained by alloying diminished. In order to better understand the effect of solute atoms on strength, samples doped with 10 at.% Ag only at the interface are prepared. These samples did not show significant strengthening, indicating that the alloying does not alter the interface barrier strength. The findings show that multilayers can reach their peak strength not only through layer thickness reduction but also through alloying additions. Alloying elements and compositions combined with layer thickness will provide a new design space for the development of high strength nanostructured metals.

11:45 AM - TC06.07.05

How to Play with Grain Size and Texture to Tune Mechanical Properties of Architectured Materials—The Case of Cu-Nb (Nano)Composite Wires

Traditionally the reversible plasticity and shape-memory effects in metals upon heating are mediated by a collective or “military” transformation and mechanisms. In initially defect free Au nanoparticles show a fundamentally different reversible plasticity and shape recovery upon indentation and subsequent annealing by guided surface diffusion.The combination of annealing experiments, a diffusion model and atomistic simulation reveal consistent that slip traces from the dislocation mediated plasticity at the free surfaces provide a network of fast diffusion pathways. This fast diffusion pathways at terrace ledges guide the mass transport during carefully chosen temperature regime back to the indent.At the same time the already initially non-equilibrium particle shape is maintained, whereas the indent disappears.This self-healing phenomena and the kinetic regime (annealing temperature, annealing time) is discussed as competition of terrace ledge diffusion, Schwoebel barrier crossing and sink-strength of the indent and the necessary particle size regime for the plasticity to reside as imprints on the particles facets without a substantial defect-network in particle's interior.

Dislocations are ubiquitous in metals where their motion presents the dominant and often the only mode of plastic response to straining. Over the last 15-20 years computational prediction and understanding of fundamental dislocation mechanisms of plastic response in metals has been in the crosshairs of the materials modeling community. The typical multiscale approach relies on atomistic molecular dynamics (MD) simulations to obtain dislocation mobility functions and local evolution rules that are then employed in mesoscale dislocation dynamics (DD) simulations to model crystal yield, flow, and hardening from the underlying dislocation motion. Here we present first direct atomistic MD simulations of bulk crystal plasticity that skip the DD mesoscale altogether and compute plasticity response of single crystal tantalum while tracing the underlying dislocation behaviors in all atomistic details.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Nanotwinned metals are a promising class of modern materials combining a very high strength with a high ductility and excellent electrical properties. This remarkable strength is believed to be connected to the efficiency of twin boundaries as obstacles to dislocation motion, although the exact mechanisms are still under discussion.In order to further characterize these interactions, micropillars containing single twin boundaries with different orientations were compressed with a flat punch, and subsequently investigated in the scanning electron microscope. The investigations concentrated on copper and α-brass, which is a low stacking-fault energy alloy exhibiting a high density of recrystallization twins. Coherent twin boundaries were selected from an EBSD orientation mapping of the sample and oriented by means of a custom 3D-printed sample holder. FIB-milling at these interfaces yielded micropillar specimens containing a single twin boundary. Single crystalline reference samples were obtained from the bulk of the grains located on both sides of the twin boundary. The microcompression tests allowed quantifying the influence of the twin boundary barrier on the strength of the sample. The activated glide systems were subsequently identified from slip trace analysis and STEM mapping of lamellas obtained by lift-off from the bulk of the tested micropillars. The different deformation modes will be discussed in the presentation.

The introduction of nanoscale twin boundaries (TBs) is an effective strategy to achieve exceptional combination of superior strength, ductility and resistance to fracture, fatigue and wear. It has been demonstrated that the properties of nanotwinned metals are dominated by a number of deformation mechanisms unique to the interactions between dislocations and TBs. Here, we reveal yet a new type of dislocation mechanism called correlated necklace dislocations (CNDs). This mechanism controls the strengthening of nanotwinned metals as the twin thickness is reduced to around 1 nm. The presence of a cracklike defect as the dominant dislocation source could allow the same mechanism to operate at larger twin spacings. More importantly, we demonstrate that CNDs could be responsible for an unusual, history-independent and stable fatigue behavior of nanotwinned Cu containning highly oriented nanoscale twins. Our findings call for further theoretical and experimental investigations of the unique deformation mechanisms in nanotwinned metals.

Twin boundaries can enhance the strength and sustain ductility of a variety of metallic materials with face centered cubic structures. However, twin boundaries are rare in Aluminum due to its high stacking fault energy (SFE). Previous studies show that the introduction of Ag seed layer is necessary to promote the formation of nanotwins in Al films. Here, we show that high-density twin boundaries can be introduced in Al films by tailoring the texture of films without any seed layer. We also examine the correlation between the crystal growth orientation and twin formation mechanisms by using Transmission Kikuchi diffraction and transmission electron microscopy. Furthermore, mechanical testing indicates that twin boundaries in Al appear to be stronger barriers to dislocations than conventional high angle grain boundaries.

2:45 PM - TC06.08.06

Twin Mediated Deformation of Copper Nanothin Film at High Elongation Strain

This talk will present twin mediated grain growth and grain refinement in nanocrystalline copper thin films subject to large plastic deformation. By applying a special adhesion technique, a copper thin film with a thickness of 100 nm deposited on a polyimide substrate can be elongated up to 30% of strain without apparent micro-crack formation. Systematic microstructure and texture analysis by transmission electron microscopy (TEM) based orientation mapping technique (ASTAR) reveals that grain coarsening occurs at the early stage of deformation and subsequent grain refinement follows during large deformation. This new phenomenon can be explained by twin mediated process based on our molecular dynamics simulations.

3:00 PM -

BREAK

TC06.09/PM03.06: Joint Session I: Grain Boundaries

Session Chairs

Dan Gianola

Dan Mordehai

Tuesday PM, November 28, 2017

Hynes, Level 2, Room 210

3:30 PM - *TC06.09.01/PM03.06.01

Quantifying the Commonalities in Structure and Plastic Deformation in Disordered Materials

The nonequilibrium nature of kinetically frozen solids such as metallic glasses (MGs) is at once responsible for their unusual properties, complex and cooperative deformation mechanisms, and their ability to explore various metastable states in the rugged potential energy landscape. These features coupled with the presence of a glass transition temperature, above which the solid flows like a supercooled liquid, open the door to thermoplastic forming operations at low thermal budget as well as thermomechanical treatments that can either age (structurally relax) or rejuvenate the glass. Thus, glasses can exist in various structural states depending on their synthesis method and thermomechanical history. Nanocrystalline (NC) metals, also considered to be far-from-equilibrium materials owing to the large fraction of atoms residing near grain boundaries (GBs), share many commonalities with MGs both in terms of plastic deformation and its dependence on processing history. Despite these similarities, the disorder intrinsic to both classes of materials has precluded the development of structure-property relationships that can capture the multiplicity of energetic states that glasses and GBs may possess.

Here, we report on experimental studies of MG and NC materials and novel synthesis and processing routes for controlling the structural state – and as a consequence, the mechanical properties. A particular focus will be on strategies for rejuvenation of disorder with the goal of suppressing shear localization and endowing damage tolerance. We also describe a microscopic structural quantity designed by machine learning to be maximally predictive of plastic rearrangements and further demonstrate a causal link between this measure and both the size of rearrangements and the macroscopic yield strain. We find remarkable commonality in all of these quantities in disordered materials with vastly different inter-particle interactions and spanning a large range of elastic modulus and particle size.

4:00 PM - *TC06.09.02/PM03.06.02

Predictive Equations of Motion for Grain Boundaries and Triple Junctions in Polycrystals

The equation of motion for grain boundaries (GB) and triple junctions (TJ) are commonly stated as the velocity is equal to the product of mobility and driving force, where the driving force may be associated with capillarity, jumps in the stored defect concentrations across the GB, etc. This class of equations of motion does not account for the microscopic mechanisms by which, we now know, GBs move; i.e., through the motion of disconnections (line defects within the GB). We first present a discrete disconnection model of GB migration; including how different classes of driving forces combine and how mobility depends on temperature and GB crystallography. We quantitatively validate this understanding through a series of atomistic simulations. Next, we derive a continuum equation of motion for GBs that is based on the underlying discrete disconnection dynamics mechanism. Finally, we provide a model for how TJs moves within the continuum disconnection dynamics framework, including several numerical examples of the motion of GBs delimited by TJs.

Interfacial Defects (ID) refer to atomic arrangements delimiting the boundaries between distinct phases or material domains. IDs are found in bulk polycrystals in which diffusion-less transformations (twinning, phase changes) can be activated as an alternative or complement to dislocation glide mediated plasticity. This is the case of hexagonal close packed (hcp), face centered cubic (fcc), body centered cubic (bcc) and lower crystal symmetry materials in which such transformations can be activated depending on temperature, composition, and loading conditions. Twinning corresponds to a shear transformation while phase transformation is usually also dilatational. The work to be presented aims to study the effects of collective nucleation, motion and interaction of ID on the evolution of transformed domains (twins, transformed phase), associated internal stresses, and mechanical response. To this end a novel generalized discrete defect dynamics model, capable of simultaneously describing both dislocations and disclination, is proposed. Further, a series of didactive simulations will illustrate the kinematics of disclination motion and their implication on twin growth.

The synthetic driving force method is a widely-used technique in molecular dynamics simulations to investigate the migration of grain boundaries. Its physical essence, however, has been under debate for quite some time for generating the driving force by artificially introducing some energy into the crystals. In this study, the elementary process governing the grain boundary motion under the driven motion method was explored by applying a varying synthetic driving force that increases from zero at a constant rate, which is in contrast to a constant driving force that is usually applied in past studies. With this method, it was found that a rate-controlling process, i.e., disconnection nucleation that has been reported before to dominate the physical grain boundary motion coupled to an applied shear, also operated for grain boundary motion caused by the synthetic driving force. Furthermore, the disconnection nucleation mediated process was also found to cause a strong size dependence and transitions of grain boundary motion modes at different temperatures. It is hoped that with this study, the synthetic driving force method in studying grain boundary motion can be used with more confidence in its physical essence and a universal mechanism can be proposed to explain grain boundary motion in materials despite how it is caused.

Nanomaterials possess high surface/volume ratio and surfaces play an essential role in the size-dependent material properties. The present presentation introduces the surface induced size-dependent ultimate tensile strength, size- and temperature-dependent Young’s modulus, and size-dependent thermal expansion coefficient of thin films. The surface eigenstress model is further developed to analytically formulate the surface induced material properties. Thin films are taken as typical examples to illustrate the physical picture clearly. First-principle calculations and molecular dynamics simulations are conducted and the results agree perfectly with the derived equations. The ultimate tensile strength of Au thin films increases when the film thickness decreases, indicating a typical the-thinner-the-stronger phenomenon, whereas the Si thin films exhibit the-thinner-the-weaker behavior. The size-dependency of tensile strength of thin films is caused by the surface induced electron redistribution and then the change in the bonding strength between the surface atoms. In addition to surface eigenstress and surface modulus, thermal coefficients of surface Young’s modulus, and surface thermal expansion coefficient are also introduced here to analytically express the size- and temperature-dependent Young’s modulus, and size-dependent thermal expansion coefficient of thin films. The values of these surface properties are determined from the atomistic calculations. The newly developed surface eigenstress models are able to attack the similar problems in other types of nanomaterials and the methodology developed is also capable to study mechanical behaviors of interfaces.

Cubic Silicon carbide (3C-SiC) has excellent physical properties, such as thermal stability, high hardness, and superior corrosion resistance. Studies of silicon carbide films have attracted interest to be surface treatment of graphite substrate with applications in semiconductor, LED and chemical corrosion components. 3C-SiC films are grown on graphite by hot-wall chemical vapor deposition (Hot-Wall CVD) using SiCl4 and CH4 as precursors. We perform a comprehensive experiment to change deposition temperature, pressure and flow rate of SiCl4. We also investigate microstructures and mechanical properties of 3C-SiC films individually. In addition, the simulation and theoretical model of Hot-wall CVD system is established to observe changes of both gas and flow fields inside the reaction chamber. The microstructure and composition of films are analyzed by scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS). The film phase has been identified by X-ray diffraction (XRD). As the temperature increases from 1130°C to 1330°C, the deposition rate of films increase rapidly from 7.4 μm/h to 33.6 μm/h, and the hardness is examined from 10.7 GPa to 22.1 GPa by nanoindentation. It has been observed that the deposition temperature influences on the hardness and deposition rate of 3C-SiC films intensely. By individually varying deposition temperature, pressure, and the ratio of precursor, the effect of these parameters on both microstructure and mechanical behavior will be discussed. Furthermore, the thermal stability and degradation of 3C-SiC films have been carried out under varied temperature conditions at 500°Cand 1000°C, respectively.

8:00 PM - TC06.10.04

Probing the Thickness Dependence of the Full Elastic Tensor of Sub-50nm Low-k Dielectric Thin Films Using Coherent EUV Nanometrology

Understanding the mechanical properties of novel nanostructured devices is of critical importance from both a fundamental and technological perspective. Ultra-thin films have a wide range of applications in the semiconductor industry, for applications in chemical protection, packaging, or enhancing conductors and insulators. Therefore, progress in the fabrication and characterization of these films is crucial. Nanofabrication techniques have pushed the limits of thin film synthesis down to atomic resolution in thickness and scalability up to wafers of 45cm diameter. However, precise metrology tools remain a bottleneck for understanding thin film physics at thicknesses below 100nm. Traditional metrology techniques face two main challenges when measuring the elastic properties of sub-100nm thin films: first, distinguishing the thin film properties from the substrate, and second, extracting more than one component of the elastic tensor.We overcome these challenges by using coherent EUV nanometrology: a novel metrology technique that utilizes coherent extreme ultraviolet (EUV) light produced from tabletop high-order harmonic generation (HHG) [1]. The short wavelength of the EUV light enables sensitivity to short wavelength surface acoustic waves (SAWs), here as small as 45nm, which can be confined exclusively to a thin film. Furthermore, our technique extracts both components of the elastic tensor simultaneously by measuring both SAWs and longitudinal acoustic waves (LAWs) in isotropic films. Finally, the ultrafast HHG pulses can resolve sub-picosecond acoustic and thermal dynamics of the nanosystem.SAWs and LAWs are launched in these films by impulsively heating periodic gratings of nickel nanolines deposited on top of the films, with a femtosecond 780nm laser pump pulse. SAWs propagate exclusively within the thin film for a sufficiently small grating period or penetrate into the substrate for larger ones. LAWs are launched within the nanowires, propagate into the film and substrate beneath, and reflect from the film-substrate interface. We probe these dynamics by employing a pump-probe spectroscopy where we monitor the diffraction efficiency of the 30nm-wavelength EUV probe beam from the nanograting on top of the thin film surface.We recently extracted the full elastic tensor of a series of sub-100nm a-SiC:H thin films with different levels of hydrogenation [2]. An unexpected trend in the Poisson’s ratio was uncovered, showing a transition from brittle to ductile at a specific network connectivity threshold value. Here, we present new measurements of the full elastic tensor of three low-k dielectric a-SiOC:H thin films with systematically varying thicknesses down to 11nm, which is the thinnest film to have been fully characterized to date. We examine the impact of the thickness and interface quality on the elastic tensor of these films.

Organic-inorganic hybrid perovskite materials have been widely used in the field of photovoltaics due to their excellent advantages such as high color purity with a narrow full-width at half maximum, high charge carrier mobility, tunable optical bandgap, and low-cost solution-based processing. Recently, many researchers have reported deformable perovskite LED, since the demand for flexible and stretchable electronics increases. Although flexibility of perovskite LED has been determined mainly through empirical methods, flexibility of perovskite materials in LEDs has not been studied systemically. Since LEDs are manufactured in the form of multilayer structure with various constituent materials in thin films, compressive and tensile strain are varies depending on the distance from neutral axis under the bending stress. It is important to measure mechanical properties and analyze fatigue resistance for perovskite, which is the weakest material in LED, because a fracture will occur at this point.In this work, fatigue resistance of perovskite materials was evaluated by crack propagation mechanism. We prepared samples for mechanical testing with different microstructure by same procedure for manufacturing perovskite LEDs. Mechanism of fatigue crack propagation in perovskite materials was evaluated by measured crack nucleation and propagation every several hundred-bending cyclic under various bending strain. We compared mechanism of fatigue-crack propagation with mechanical behavior as determined by uni-axial tensile test in SEM. We evaluated suitable microstructure for flexible perovskite LED by fatigue resistant of perovskite materials.

8:00 PM - TC06.10.06

The Effect of Solid Solution Strengthening on the Size Effect in Micro-Compression Tests

In the present study, we investigate the effect of solid solution strengthening on the size effect in face-centered cubic metallic alloys. Fe-Mn-C based alloys in solid-state solution are examined with a different solid solution strengthening contribution and compared with literature data on pure fcc metal. Our results allow to interpret the size-dependent behavior based on a quantitative consideration of the solid solution strengthening, as other material or experiment parameters are carefully controlled. The prediction of size dependent behavior using a unique size scaling exponent is statistically assessed.

The very mature, cost effective and efficient manufacturing techniques of silicon based electronics industry have leveraged its great success. Unfortunately, conventional silicon based rigid and brittle electronics lack the ability to be conformal and flexible as demanded by certain novel and rising applications such as bio-implantable and wearable electronics. Fortunately, on the other hand, the rigidity of a material is a function of its thickness, thus ultra-thin silicon sheets have shown natural flexibility. Moreover, even rigid materials can become stretchable through structural modifications. A thin sheet of silicon designed into a pattern can be not only flexible but also stretchable. As a result, silicon has become a very strong material candidate for flexible and stretchable electronics. Recently, silicon spiral-based structures have been proposed as ultra-stretchable platforms [Huang K. et al., IEDM, 217- 220, 2007; Rojas J.P. et al., Appl. Phys. Lett. 105, 154101, 2014]. Inspired by this and the concept of fractals (structures within structures), we present an all-silicon compound structure based on a double arm spiral, where a continuous string of serpentines replaced the straight arms. Moreover, to reduce the stress localization at the start and the end of the spiral-serpentine arms, horseshoes structures were added at these points. Using finite element analysis, it was found that a compound structure containing spiral, serpentines and horseshoes gives the optimum results. A stress/strain reduction of around 55% was observed for this compound spiral-serpentine-horseshoe structure compared to a simple spiral structure [Mutee U.R., Rojas J.P., Extrem. Mech. Lett. 15, 44-50, 2017]. Moreover, a much more even stress distribution along the arm was observed compared to simple spiral where the stress was localized at the start and beginning of the arms, critical points for spiral structure. Further reduction in stress value along the arm would be even possible by replacing more serpentines with horseshoes along the arms but at the cost of more area. Here we report the fabrication of this compound structure using a thick silicon-on-insulator (SOI) (50 mm Si / 10 mm SiO2) wafer and standard microfabrication techniques. The fabrication process consist of four simple steps; Hard mask deposition, patterning, deep reactive ion etching, release. The fabricated spiral structure was tested to validate the simulation and fabrication process, achieving ~470% stretchability. It is to be noted that the stretchability of these structures can be tuned by simply selecting the number of arms’ turns. We believe these silicon-based compound structures represent an important step in the development of highly stretchable electronic systems. In future, the complete mechanical characterization of the fabricated structures will be completed to identify their stress/strain, and the maximum stretchability that could be achieved with these compound structures.

Flexible, bendable, stretchable devices represent the future of electronics for applications ranging across disciplines, including health monitoring and therapeutics, light-emitting devices, solar photovoltaics, sensors, and communications technology. Silicon nanocrystals (SiNC) exhibit size-dependent physical and photoluminescent properties which makes them interesting candidates for use in optoelectronic devices. Since the system of a highly deformable substrate coated with a thin film on SiNCs can be modeled as a multilaminated structure, it is important to know that a bilayered system with large differences in component thickness and/or mechanical properties that is subject to large deformations is susceptible to formation of mechanical instabilities. A combination of flexible polymers and thin film silicon nanocrystals with elastic-instability-induced microstructural deformation have great potential to be used in photonic metamaterial applications. By employing continuum mechanics, a mathematical framework can be developed to describe and predict the onset of these instabilities. However, while semiconductor nanocrystals have been previously used in stretchable devices like LEDs, there have been few studies to evaluate the mechanical behavior of silicon nanocrystals on stretchable substrates.Characterizing and measuring surface instabilities on a thin film nanocrystalline layer is an important step for development of tunable optical metamaterials. Therefore it is required to define an appropriate constitutive law for mechanical behavior of silicon nanocrystals and estimate mechanical properties of the nanocrystal layer, as well as to increase our understanding of how the nanocrystal behavior changes in relation to elastic properties of the substrate.We present a novel method for measuring the mechanical properties of a thin silicon nanocrystal film over an elastomer. The method exploits the formation of plane strain bifurcations on elastic layered structures subject to finite bending. This technique involves measuring the critical bifurcation angle for a multi-layered elastic structure on the application of a pure bending boundary condition. These findings can then be used to evaluate the material characteristics of the silicon nanocrystal layer using neo-Hookean properties as a first approximation if the mechanical properties of the base layer is known.

8:00 PM - TC06.10.09

An Atomic Simulation—Intraplaner to Interplaner Fracture Behavior of Multi-Layer MoS2

MoS2 emerged as the substitutional material for graphene used in semiconductor recently, compared with graphene, it has a direct bandgap which can be adjusted by varying its thickness. MoS2 membrane represents the critical hierarchical structure which bridges the length-scale of monolayer and bulk material architectures. Using MoS2 membrane in nanomechanical, or nanoelectromechanical systems (NEMS) involves the moving, rotating, compressing and stretching motions of objects. So, the mechanical properties including the fracture mechanism were considered as the fundamental aspects of MoS2 membrane to investigate. The direct experimental measurements of the stretching properties of MoS2 membrane is highly difficult, thus, the computational or theoretical methods should be carried out. Among all the theoretical approaches, ab initio technique is more accurate but with reachable system size and limited time scale. Molecular dynamics simulations can be more helpful in this respect, it can provide comparable size for experimental sample and give straightforward explanation for fracture modes by measuring the fluctuations and movements of atoms during a certain time interval. So, in this work, a set of Molecular Dynamics simulation was carried out to simulate the tensile loading for MoS2 membranes with different thickness. Tensile testing was performed using the LAMMPS code with modified REBO potential. This work provides benefit reference about effects of external elements such as layers, thickness, defects, strain rates etc., on the mechanical properties of the of MoS2 membrane and the threshold range it can bear in practical applications. Additionally, the uniaxial tensile simulations of MoS2 membrane shows obvious thickness effect: thinner membrane exhibits a relatively larger fracture strength and strain than the thicker one. More importantly, the fracture modes of MoS2 membranes are various regarding to different thickness/layers, the thicker MoS2 membranes show interplanar fracture, while monolayer and few-layer MoS2 are dominated by intraplanar fracture. There exiting a typical MoS2 nanosheet fractures as the intermediate state from interplanar to intraplanar fracture. Our study provides critical insights for layered MoS2 2D material applications.

Recent studies on body-centered-cubic (bcc) metal nanopillars revealed that dislocation are more easily multiplied in bcc structure than face-centered-cubic (fcc) structure. Computational studies in both bcc and fcc nanopillars suggested that cross-slip, which occurs at the free surface, plays an important role in dislocation multiplication at small length scales. Particularly for bcc structure, the relative difference in mobility between screw and edge dislocation is critical to induce the surface-controlled dislocation multiplication. The dislocation mobility of bcc metal is strongly dependent of intrinsic lattice resistance, which is a function of temperature, and the surface controlled multiplication behavior should also be dependent of temperature. In this study, therefore, we investigated how a single pure screw dislocation is multiplied through surface-controlled multiplication in a bcc nanopillars at a different stress, temperature (10~300 K), and nanopillars size by using constant stress molecular dynamics simulation.

We chose [0 0 1]-oriented niobium and molybdenum nanopillars due to their different intrinsic lattice resistances. We created a single pure screw dislocation in a nanopillar, and applied the constant uni-axial compressive stress to drive the dislocation to move. We characterized the critical stress of dislocation multiplication as a function temperature and nanopillars size. Our results revealed the conservative increase in critical stress of multiplication with temperature, implying that surface-controlled multiplication is dependent of an absolute value of dislocation mobility as well as the relative difference in mobility between screw and edge dislocation. Also, we found that multiplication behavior has a strong correlation with the nanopillars size and the critical temperature, at which screw dislocation mobility becomes the same with edge dislocation mobility. Our simulation results will be carefully compared with experimental results of dislocation-free bcc nanopillars and dislocation-containing nanopillars at cryogenic temperatures. We will discuss how surface-controlled multiplication affects general dislocation multiplication behavior and strain burst size of bcc nanopillar. Our study will be able to bridge between experiment and computation on dislocation multiplication behavior in bcc structure at the nanometer scale.

8:00 PM - TC06.10.12

The Calculation and Analysis of Material Fracture Process Result Effected by Nano-Grain Size Energy Transfer

Classic material fracture mechanics theory is the process to produce a new interface and consume energy. Based on nano-scale study of the fracture process, pointed out that the fracture process for different materials, Nano-crystal cell in material can absorb energy and release its stored energy， it exists two possible changes. The fracture process of speed, size, and force effects would casue different change directions. Forby Nano-crystal cell of different kinds of materials，two types energy process possibilities are different, increases the complexity of the fracture, fracture result diversity and random. The paper calculated and discussed the new mechncial according to different situations.

This study reports the evaluation of vibration test of the Ti/Au micro-cantilever for MEMS (micro-electro-mechanical-systems) devices, such as MEMS accelerometer. Gold material which has the feature of high density is advantageous in lowering the Brownian noise in MEMS accelerometers [1]. In order to apply gold as movable structures in MEMS devices, investigating mechanical strengths of the material is needed. Thus, we have reported the structure stability of the Ti/Au micro-cantilever [2].The vibration test is needed to understand behavior of the material’s mechanical strength under a long-term vibration. In this study, the vibration test was carried out under the conditions of a cycle number in the range from 103 to 107, the frequency of 10.0 Hz, and the acceleration of 1.0 G (1 G = 9.8 m/s2) respectively. After the test, we measured surface profiles of the Ti/Au cantilevers by a 3D optical microscope. Length, width and thickness of the micro-cantilevers were 50 ~ 1000 μm, 5 ~ 20 μm, and 3.1 ~ 40.4 μm, respectively.After 107 cycles of the vibration test, no significant deformation nor crack was observed in all of the micro-cantilevers. The results revealed that structural stability of the multiple Ti/Au layered design is high and advantageous to the device reliability. A downward deflection at tip of the micro-cantilevers was observed in all of the micro-cantilevers after the vibration test. Magnitude of the tip deflection was smaller for the micro-cantilevers with larger overall thickness. This finding followed well with the Euler–Bernoulli beam theory, where structure stability of cantilevers is enhanced with an increase in the thickness.

Enhancement in the performance of MEMS (micro-electro-mechanical-systems) accelerometers is achieved by replacing the Si-based movable components to Au-based micro-components [1]. Sensitivity of the MEMS devices is improved by reducing the Brownian noise through usage of a high density material, such as Au. In order to improve the structure stability for applications as movable structures in MEMS, a structure composed of multiple Ti/Au layers is proposed [2]. On the other hand, there is still no report on the effect of temperature on structure stability of the micro-structures. Therefore, temperature dependence of structure stability of the Ti/Au micro-cantilever was studied.The micro-cantilevers were consisted of one to two layers of the Ti/Au layer. Illustration of the cantilever is shown in previous studies [2]. Thickness of the Ti layer was fixed at 0.1 μm. Thickness of the Au layer was 3 or 12 μm. Combinations of the Ti and Au layer and numbers of the Ti/Au layer were varied depended on the desired total thickness for a specific function. Length and width of the micro-cantilevers used in this study were ranged from 50 to 1000 μm and from 5 to 20 μm, respectively. The temperature dependence test was started from 20 °C, then heated up to 100 °C, followed by cooling down to -50 °C, and heated up back to 20 °C. A step-size of 10 °C and a rate of 1 °C/min were used during the heating and cooling processes. The temperature was kept for 5 min when reaching the specific temperature in each step before going to the next step. The structure stability was evaluated by observing tip deflection of the cantilever by a 3D optical microscope. FEM (finite element method) simulation using COMSOL multi-physics software was also conducted.After the test, an upward tip deflection of 0.17 μm was observed from OM observation for the cantilever composed of a single Ti/Au layer and total thickness of 10.1 μm, length of 400 μm and width of 20 μm. The cantilever with two Ti/Au layers, total thickness of 15.2 μm, length of 400 μm, and width of 20 μm showed an upward tip deformation of only 0.02 μm. On the other hand, FEM simulation results of all the cantilevers showed a downward deflection when the temperature was increased to 100 °C, an upward deflection when the temperature was decreased to -50 °C, and an downward deflection when the temperature was returned to 20 °C. Magnitude of the tip deflection, both downward and upward, was smaller for the cantilever with more numbers of the Ti/Au layer. The test results confirmed the multiple Ti/Au layer design has a tolerance for temperature change.References[1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett., 104 (2014) 074102.[2] M. Teranishi, T.F.M. Chang, C.Y. Chen, T. Konishi, K. Machida, H. Toshiyoshi, D. Yamane, K. Masu, M. Sone, Microelectron. Eng., 159 (2016) 90-93.

Precise knowledge of the mechanical properties of free-standing, micron-sized structures is of critical importance for the reliable design of modern micro devices. One particular example are stretchable electronic circuits where such structures provide the electrical interconnectivity between the individual building blocks of the circuit. The reliable function of these circuits depends on the ability of the interconnects to repeatedly withstand large strains while maintaining minimal footprint. The small size and the high stretchability of the free-standing microstructures makes conducting experiments with them challenging since improper handling during mounting can damage or destroy them. These structures are usually made using microfabrication technologies and need to be transported from the fabrication site to the test setup thus increasing the risk of damage. To address these issues the authors propose an approach where the microstructures are fabricated on a flat silicon substrate using standard microfabrication technology. The fabrication flow results in the manufacturing of a number of chips containing test microstructures suspended over a gap between two silicon beams. The beams together form a single chip. Since the delicate test microstructures are integrated in a monolithic chip they can be easily moved from the fabrication facility and mounted in the test setup without risk of damaging or destroying them. Once the chip is securely mounted in the test setup, it is split into two independently movable beams by a controlled fracture. The separated beams can then be displaced independently over large range of strains with great precision allowing for a testing the structures over very large strains.

Chips containing free-standing aluminum microstructures have been fabricated and successfully mounted on a micro tensile test stage. Tensile tests have been performed while the deformation of the microstructures has been observed with visible light microscopy, scanning electron microscopy and white light interferometry. Since the microstructures are to be used as electrical interconnects leads for resistance measurement have also been incorporated in the design, allowing the electrical resistance to be tracked while the structures are being tested.

The currently presented work demonstrates that the proposed approach can be successfully used to probe the behavior of structures at micron scale. The approach can also be used to characterize microstructures with different dimensions and shapes made from a wide variety of materials using a number of analytical tools providing experimental input for simulations.

The search for new ultra strong materials has been a very active research area [1]. With relation to metals, a successful way to improve their strength is by the creation of a gradient of nanograins (GNG) inside the material [2]. Last year, R. Thevamaran et al. [3] propose a single step method based on high velocity impact of silver nanocubes to produce high-quality GNG. This method consists in producing high impact collisions of silver cubes at hypersonic velocity (~400m/s) against a rigid wall. Although they observed an improvement in the mechanical properties of the silver after the impact, the GNG creation and the strengthening mechanism at nanoscale remain unclear. In order to gain further insights about these mechanisms, we carried out fully atomistic molecular dynamics simulations (MD) using the well-known and tested Embedded-Atom Method potential (EAM). In particular, we want to investigate the atomic conformations/rearrangements during and after high impact collisions of silver nanocubes at ultrasonic velocity. Our results [4] show that the high velocity impacts induce an extensive deformation process in the structure through amorphization of highly stressed regions, located at the cube bottom. It was possible to see observe the co-existence of polycrystalline arrangements after the impact, formed by core HCP domains surrounded by a FCC ones; this indicates that during the impact the cubes undergo a phase structural transition from FCC to HCP stacking in some regions. Experimentally, the silver strengthening is attributed exclusively to the GNG creation inside the material. However, the core HCP arrangements observed in our simulations could also contribute to explain the structural hardening, since our stress-strain simulations show that the mixed HCP/FCC domains exhibit higher strength values than pure FCC silver ones.[1] P. Hazell, Armour: Materials, Theory, and Design (CRC Press, 2015).[2] T. H. Fang, W. L. Li, N. R. Tao, K. Lu, Science v331, 1587–1590 (2011).[3] R. Thevamaran, O. Lawal, S. Yazdi, S.-J. Jeon, J.-H. Lee, E. L. Thomas , Science v354, 312-316 (2016).[4] E. F. Oliveira, P. A. S. Autreto and D. S. Galvao – submitted.

8:00 PM - TC06.10.18

Crack Manipulation Induced by Stress Localization and Their Applications

The cracks formed usually exhibit random and irregular morphologies. They are regarded as one type of deformations and are mostly considered as drawbacks and/or defects. Controlled cracks, however, can be a valuable technique for processing hard materials. Recently, various methods for controlling cracks have been suggested. These methods involve the direct creation of cracks by physical contacts using micro-tips or notches, which has extensively been studied.Herein, we report a new method of manipulating the cracks combining TiO2 nanoparticles (NPs) with soft imprinting. We demonstrate that crack manipulating is determined by the thickness of film layer, NP size, heating rate and explain the results obtained based on fracture mechanics. Furthermore, by controlling the film thickness, we analyze the relation between the thickness and fractured area depending on the surface treatment effect. Since our experimental system utilizes square pyramid patterns with square pixels, it allowed us to quantitatively analyze the controlled cracks that were not experimentally realized in the previous crack research. It also can demonstrate models for simulations.Finally, by detaching these divided mesoporous structures from low adhesive substrates, mesoporous TiO2 wires and particles of uniform shape and size were successfully obtained. Comparing to the classical microstructure fabrication methods, the approach introduced here is a simple, low-cost process with dimension, material, and pore size readily tuned by varying the type of nanoparticles and patterns used.

8:00 PM - TC06.10.19

Elastic Properties of Solids under Extreme Conditions—High Pressures and High Temperatures

There is widespread interest in the elastic properties of solids at elevated temperatures and high pressures. Much of the impetus for current research in this area has arisen from the need to understand phase transitions and the behavior of the elastic properties of functional, superhard materials and hard coatings at extreme conditions. Laser ultrasonics (LU) appears to be most appropriate technique for determination of the acoustical properties of very small non-transparent solids and thin films at high temperatures and high pressures. For instance, high temperature measurements are usually conducted inside a vacuum furnace, which makes it difficult to apply conventional techniques to study the effect of high temperature on the elastic properties of small micron-sized specimens.In this report, we describe the development of a multi-functional system for high pressure, high temperature measurements. The system is equipped with a LU set up, Raman device, and laser heating system (LU-LH). Laser ultrasonics combined with LH in a diamond anvil cell (LU-LH-DAC) demonstrated to be an appropriate technique for direct determination of the acoustical properties of solids under high pressure and high temperatures. The use of lasers generating nanosecond acoustical pulses in solids allows measurements of the velocities of shear and longitudinal waves in iron up to 50 GPa. The system is unique and allows us to: (a) measure shear and longitudinal velocities of non-transparent materials under high pressure and high temperature (HPHT); (b) measure temperature in a DAC under HPHT conditions using Planck's law; (c) measure pressure in a DAC using a Raman signal; and (d) measure acoustical properties of small flat specimens removed from the DAC after HPHT treatment.We present results on measurements of shear and longitudinal wave velocities in iron under high pressure up to 52 GPa, and of the behavior of the velocity of a Rayleigh wave in a PtRh alloy at high temperatures up to 1500 K. Finally, we demonstrate, for the first time, that the LU-LH-DAC technique allows measurements of velocities of the skimming waves in iron at 2580 K and 22 GPa. The ability to detect an LU signal at 2600 K was an unexpected surprise. The maximum intensity of the black body radiation at 2600 K is around 1.1 micron. This is very close to the wavelength of the pump laser (1.06 micron) used for the excitation of the acoustical waves in iron. Therefore, excitation and detection of the acoustical waves by the laser ultrasonics technique is possible even for very bright objects.

Generally, permanent strain remains in a metallic material after the unloading process when the metallic material is deformed beyond the yield point and enters plastic deformation region. On the other hand, shape memory alloys (SMAs) can recover to the original shape after plastic deformation occurred simply by conducting unloading process in a certain temperature range. This phenomenon is called superelasticity. SMAs possessing superelasticity are promising materials to be applied in many fields. However, in the conventional SMAs, recovery of the strain or the shape is slow because the recovery speed highly depends on heat conduction. To overcome this problem, ferromagnetic shape memory alloys (FSMAs) are developed [1]. FSMAs have attracted a lot of attention because they generate large magnetic field induced strain combined with relatively high response frequency. Therefore, FSMAs are ideal materials for micro-actuators and sensors need fast response speed [2]. NiFeCoGa is an example of FSMAs, and it exhibits a very small hysteresis loop in the stress-strain curve with a composition of Ni50Fe19Co4Ga27 [3]. FSMAs showing a small hysteresis loop are also advantageous in fatigue applications.When SMAs, including FSMAs, are used in micro-scale, influence of a single crystal grain with a specific crystal orientation on the superelastic effect would be more significant than that in the bulk specimen. Thus, it is necessary to evaluate the superelastic effect in micro-scale by conducting micro-compression test of a single crystal SMA micro-specimen for applications in miniaturized devices. Furthermore, it is necessary to carry out the micro-compression test at elevated temperatures because mechanical properties of SMAs are highly dependent on the temperature.In this study, micro-mechanical properties of Ni50Fe19Co4Ga27 were evaluated by micro-compression tests at various temperatures. The micro-specimen was a micro-pillar fabricated by focus ion beam [4]. Austenite transformation finish temperature of the FSMA evaluated in this study was estimated to be about 270 K [3]. Hence the test temperature was varied from 298 to 393 K. The experimental results showed that the pillar exhibited superelastic behavior at all test temperatures. Furthermore, the temperature dependence of the stress for martensitic transformation start showed 0.52 MPa/K, which is relatively smaller than those of other SMAs.

Residual stresses plays an important role in defining the properties of materials. Depending upon the sign, magnitude and distribution of stresses, the effect could be either beneficial or detrimental and hence an important performance indicator. The present study focus on the assessment of residual stresses in nanostructured and amorphous materials using incremental focused ion beam (FIB) milling, combined with high-resolution in situ scanning electron microscopy (SEM), a full field strain analysis by digital image correlation (DIC) and finite element modelling (FEM). Residual stresses are evaluated on different material/coating systems to highlight the versatility of the technique. The materials/coating system chosen for this study are; a) Amorphous silicon nitride (Si3N4) coating with a thickness of ~ 200 nm, b) Ni-B coating with a thickness ~ 40 µm and d) Carbon fibers embedded in polyether ether ketone (PEEK) matrix. Discussion on residual stresses are made in terms of both average and through thickness stress profiling. Results shows that the method is not only applicable to wide range of materials but also an affordable and accessible technique for industry.

Buckling can be found in our life such as surfaces of human or animal skins, dried fruits, etc. Until now, vertical buckling phenomena have been studied and exploited for various applications such in an optical grating, microfluidics, adhesives, and so on. Lateral buckling of high aspect ratio wall structures has been demonstrated by swelling of hydrogel line patterns, or compressive stress originated from a coating of reactive films on laterally free standing structures. However, the buckled structures were fixed and could not be changeable. In this talk, we propose a responsive concept of lateral buckling of wall structures. By applying compressive stress, the walls are buckled laterally and returned to the original shapes by releasing the stress. We explain the origin of the shapes by scaling theory as well as finite element methods. Also, an optical application by the lateral buckling is demonstrated.

Micro-mechanical testing has been utilized to evaluate a synthetic nuclear-grade graphite that has microstructural features, specifically pores and crack-like defects, that range in dimension from nanometre to millimetre scales. This graphite material is not only scientifically interesting, but has an industrial imperative as it is used in nuclear reactor cores and acts as a neutron moderator and structural component for fuel and control rod channels. Accordingly, it is essential to assure the structural integrity of these graphite composite components, especially after neutron irradiation, and understand their mechanical behaviour over multiple length-scales. The output from the micro-mechanical tests are used as input to a multi-scale computer model to predict potential fracture of graphite at macro-scale.

Specifically, micro-size beam samples with cross-section of a few micrometres up to tens of micrometres have been tested in situ inside a SEM chamber to investigate the deformation and fracture mode of this graphite material. Such tests have also been performed on samples subjected to neutron irradiation (up to a dose of 7 dpa at 700°C) to study the different deformation mechanisms after service. Combined with macro-scale experimental tests, the mechanical properties, such as flexural strength and elastic modulus, have been derived for this graphite material. A multi-scale microstructure-based computer model was then used to correlate the mechanical behaviour of graphite over micrometre to millimetre length-scales. It is emphasized that due to the limited availability of surveillance samples, in situ micro-mechanical testing is of paramount importance as it correlates the local microstructure to mechanical properties; as such, it permits the prediction from computer models of the mechanical behaviour of nuclear graphite at the scale of macro-size components with a high degree of confidence.

In order to produce long-lasting components and devices the mechanical deformation mechanisms of coatings and surfaces are essential to understand. In this presentation, two examples are presented that show how coating and surface engineering result in improved mechanical stability. On a ductile substrate like titanium the usability of diamond-like carbon (DLC) coatings is often limited by the cohesive and adhesive failures as well as by contact damage creation. However, by applying a 15 nm tantalum nitride seed layer on the titanium substrate the ductile body-centered cubic alpha-tantalum phase can be triggered to growth. Due to plastic deformation of the alpha-interlayer the contact damage creation can be remarkably reduced. In addition, the fracture strength of the DLC coating gets significantly increased for a DLC coating thickness <1µm. In the second case, long-term mechanical failure mechanisms were analyzed on untreated and surface mechanical attrition treated (SMAT) 304 stainless steel. Upon repeated frictional sliding the SMAT surface shows a lower residual depth, pile-up height and friction coefficient. Furthermore, at low cycle numbers the frictional sliding tracks of the untreated 304 stainless steel meander and form slip bands adjacent to the sliding track, which is not the case on the SMAT surface.

The influence of orientation on the creep rupture properties of a single crystal superalloy DD6 undermultiaxial stress was carried out at 980 �C and 400 MPa. A circumferential V-type notched specimen hadbeen designed to investigate the effect of multiaxial stress state on the creep behavior. It was found thatthe creep lifetimes of the [111] oriented notched specimen were slightly longer than that of the [011]orientation and 1.36 times longer than that of the [001] orientation. The computational results showedthat the strain and damage distribution revealed fourfold symmetry, double symmetry and threefoldsymmetry for [001], [011] and [111] orientation respectively, which was confirmed by experimentalobservation. At high temperature the creep anisotropy in three different orientations exhibited mainly inprimary and secondary creep stages. Through the study of notched specimens by SEM, the morphologyevolution of g0-phase proved that the directional coarsening was strongly dependent on the localeffective stress and the direction of the local max principal stress with respect to loading axis. Fracturemorphology displayed uneven cleavage configuration with multi-level feature, and the cleavage planesparallels (001), (011) and (111) crystal plane for [001], [011] and [111] orientation, respectively. Thecleavage plane, which is attributed to the cracks propagated along the interfaces of g/g0phases, displayedsquare-like, rhombus-like and hexagon-like feature for [001], [011] and [111] orientation, respectively.Due to the higher density in dislocations of {111} planes, it is more easily as the secondary crack when thecrack reach the {111} planes, which is thought to be the main reason of forming different feature onprimary cleavage plane.

Since the early 1990s, coatings have been routinely characterized using nanoindentation, and scratch testing in order to optimize their mechanical properties.Most of the current international standards were written during the same period and unfortunately most users of mechanical testers only compare values between hardness, elastic modulus and critical loads but these may be insufficient for a good understanding of a coating design.Optimization of a coating remains a challenge due to the many influencing variables such as coating adhesion, yield and wear mechanism, thermal and mechanical stresses, etc.By the use of measured data from a Nanoindentation and scratch tests, a physical calculation of spatial stress profiles is simulated considering realistic material properties.Typically, in order to study the film properties during a scratch test, the tip radius is carefully chosen to locate the maximum stress in the film or at the interface. This new procedure switches the focus of adhesion study from traditional critical loads, inherently tip dependent, to a study of stresses at the interface independently of the tip radius.To illustrate the features of this new method, various examples of characterization on coated samples will be presented, showing the flexibility of the technique for different situations.

The application of body centered cubic (BCC) refractory metals is usually limited by the low temperature brittleness, which is intrinsically linked to the limited screw dislocation mobility. We present a combined ab-initio and molecular dynamics study on the role of impurity interstitials on the dislocation mobility and the implications on the brittle-ductile transition.The interaction forces between dislocation and impurities are computed for the kink-pair nucleation and kink-drift to predict strengthening contribution additionally to the kink-pair nucleation limited mobility below the Knee temperature. Continuum solute-drag models informed by atomistic simulations are used to predict experimental temperature regime for solute-drag strengthening. The role of dislocation core contribution compared to the elastic interaction is discussed and compared to recent nanoindentation experiments of high purity Cr with temperature-dependent hardness, activation volume and activation energy.

Cross-slip is a dislocation mechanism by which screw dislocations can change their glide plane. This thermally activated mechanism is an important component in dislocation-based materials modelling and understanding the activation barrier for cross-slip in a general stress condition is essential. In this work we employ a line-tension model for cross-slip of screw dislocations in Face-Centered Cubic (FCC) metals in order to calculate the energy barrier when stress is applied on the dislocation. We show that the application of Escaig stresses on both the primary and the cross-slip planes varies the typical length for cross-slip. In addition we quantify the asymmetric partial dislocations bow out into the cross-slip plane when Schmid stress is applied on the cross-slip plane. Finally, we propose an Escaig and Schmid stress-dependent closed-form expression for the activation energy for a cross slip in a large range of stresses without any fitting parameters. This analysis results in a stress-dependent activation volume, equal to the typical volume of the atoms in the dislocation segment that cross-slipped. The expression proposed here is shown to be in agreement with previous models, and to capture qualitatively the essentials found in atomistic simulations. The activation energy function can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality

In situ nanomechanical studies have advanced rapidly in recent years, from compression to tension and bending. New loading modes can be achieved through operation of a 2D transducer (with normal and lateral axes) to study combined normal and shear stress states during scratch testing. This allows for in situ study of a diverse set of deformation and failure mechanisms, such as film delamination, along with friction and wear for both industrial applications and fundamental studies. This can be done utilizing two newly developed tools, one for the SEM and one for the TEM, to enable correlation of mechanical data to observations of mechanisms at different length scales. Several case studies will be presented which address some of these new capabilities. First, in situ TEM of hard disc drive multilayer film stack, where DLC delamination, stick slip flattening of asperities and grain rotation in the magnetic recording layer are observed. Secondly, in situ TEM wear mechanisms in Olivine single crystals are explored under reciprocating scratch conditions. For the SEM, accomodation of plastic deformation in a two phase eutectic lamellar high entropy alloy and dry lubrication of steel-on-steel contacts will be addressed. These studies illustrate the broad variety of applications for such tools while demonstrating their capabilities.

Carbon nanotube (CNT) has attracted extensive research due to its extremely high tensile stress and elastic modulus (at around 150 GPa and 1TPa), high thermal and electrical conductivity. Yet to maintain the outstanding mechanical properties of CNT for micro and macro-scale CNT structures remains a great challenge. CNT yarns are one of macroscale CNT structures and can be prepared via directly drawing or spinning from CNT arrays, which can be used for many engineering applications. Here, we investigated the mechanical properties of CNT yarns at micro and nanoscales. In situ micro-tensile tests were firstly carried out inside a SEM to investigate their deformation and fracture mechanisms. We investigated the effects of diameters and twisting angle on the mechanical properties of the CNT yarns. We also carried out in situ TEM study of the CNT bundles to reveal their interfacial strength and failure mechanisms.

Hydrogen embrittlement (HE) has been known as a longstanding threat to the integrity and safety in structural metals, among which iron and steels attract the most attention due to its wide application in manufacturing industry and infrastructure. In the past century, multiple mechanisms have been proposed for the understanding of HE. Among them, hydrogen enhanced localized plasticity (HELP) and hydrogen enhanced decohesion (HEDE) are the two most discussed mechanisms. Researchers who support HELP mechanism believe that hydrogen can ‘lubricate’ dislocation motion, inhibit cross-slip, and promote slip planarity. Nevertheless, those who support HEDE suggest that hydrogen generally generates no ‘lubrication’ to dislocation motion, instead it simply lowers the bonding strength so as to facilitate cleavage-like failure. By carrying out in situ quantitative mechanical testing on sub-micron scale α-Fe samples in a state of the art environmental transmission electron microscope (ETEM), we demonstrated that hydrogenated samples show more localized plastic slip bands during compression and bending tests. In addition, quantitative measurements suggest that the activation stress of individual dislocations is lowered remarkably and the motion amplitude of these individual dislocations increases obviously given the sample being exposed to a 2 Pa level hydrogen. The effect of hydrogen on dislocations are reversible after the hydrogen source is cut off. More detailed analysis on dislocation behavior after hydrogen exposure will be discussed during this talk.

Glancing incidence X-ray diffraction was used to study W polycrystalline textured films and obtain the depth profiling of residual strain. The values of the strain averaged over different penetration depths beneath the film surface are obtained by asymmetric diffraction from crystal planes which are inclined to the film surface. The actual strain (and stresses) at different depths under the surface are extracted from the average stresses using the inverse Laplace transform method. This work was supported by grant Investissements d’Avenir of LabEx PALM (ANR-10-LABX-0039-PALM) and by the Region Ile-de-France in the framework of DIM Nano-K.

A new method combining micro-X-ray computed tomography (μXCT) and volumetric digital image correlation (V-DIC) in conjunction with in-situ mechanical testing was used to probe three-dimensional (3D) deformation behaviour in fiber reinforced composites. Intrinsic microstructural features such as fibers, pores and metal inclusions enabled accurate volumetric strain calculation of dense fiber reinforced composites using V-DIC without the need for high contrast additives. Deformation calculated with V-DIC was employed to determine variation of local mechanical properties. Unique deformation mechanisms such as internal interfacial shear and microstructure-dependent local buckling were observed in situ. Combined μXCT and V-DIC were shown to be effective for understanding 3D microscale deformations in composites.

Here we present high-frequency cyclic loading studies of FCC microcrystals to quantify the effect of crystal size on the evolution of persistent slip bands, subsequent crack initiation, and fatigue life. Experimentally, the cyclic loading is imposed using the high frequency actuator dynamics of a nanoindenters. The changes in the dislocation microstructure, and crack initiation and propagation in the microcrystals are monitored by observing changes in the beam’s dynamic stiffness and SEM imaging.

9:00 AM - TC06.11.02

In Situ SEM High Cycle Fatigue Investigation of Small Fatigue Crack Growth in Ni Microbeams

Small-scale fatigue is an active research area due to the widespread use of metallic films and micrometer-scale structures in applications such as flexible/stretchable electronics, micro and nano electromechanical systems (MEMS and NEMS), and microelectronics. In addition, the early growth of fatigue cracks comprises a large portion of the high cycle fatigue life in polycrystalline metals, and as such requires further investigation to fully understand the microstructural parameters affecting its variability. This work presents an advanced small-scale, in situ scanning electron microscope (SEM) fatigue testing technique to characterize the fatigue behavior of electroplated Ni microbeams (with an ultrafine grained microstructure) subjected to high / very high cycle fatigue loading conditions. The fatigue devices consist of MEMS microresonators that are driven at resonance inside the SEM, leading to fully-reversed loading of the microbeams at a frequency of ~8 kHz. The fatigue damage leads to a decrease of the microresonator’s resonance frequency and is measured as a metric to quantify the crack growth rates. In addition, the in situ SEM observations allow direct evaluation of fatigue crack nucleation and propagation. The stress-life fatigue curve obtained inside the SEM (vacuum conditions) highlights over three orders of magnitude increase in fatigue life with respect to the fatigue curve obtained in laboratory air. This talk will discuss the origins of the increased fatigue life based on in situ SEM observations of extrusion formation, plastic deformation localization, and fatigue crack nucleation and growth within grains. The coupling of this experimental technique with 3D crystal plasticity finite element models provides further understanding of the mesoscale parameters that drive irreversible deformation at a grain level.

9:15 AM - TC06.11.03

Designing Microcompression High Cycle Fatigue Tests up to 10 Million Cycles

Nanomechanical tests are moving beyond hardness and modulus to encompass host of different mechanical properties like strain rate sensitivity [1, 2], stress relaxation [3], creep, and fracture toughness [4] by taking advantage of focused ion beam milled geometries. Adding high cycle fatigue to this list will be useful to extend the gamut of properties studied at the micro/nanoscale. However, this presents inherent challenges like low oscillation frequencies, long duration of tests and large thermal drift when attempted with standard indenters. This presentation will report the development of micropillar compression-compression high cycle fatigue tests going up to 10 million cycles. This has been made possible by the development of a novel piezo-based nanoindentation technique that allows accessing extremely high strain rates (>104 s-1) and high oscillation frequencies (up to 10 kHz). The associated instrumentation and technique development, design of the fatigue tests at the micron scale, data analysis methodology, experimental protocol and challenges will be discussed. Validation data on single crystal silicon, a reference material, will be presented to demonstrate the reliability of the designed high cycle fatigue tests. Finally, case studies of compression-compression high cycle micropillar fatigue on nanostructured materials will be presented and their results will be discussed in light of existing literature data, particularly the operative deformation mechanism(s). The convolution of time dependent plasticity in such tests will also be addressed. It is hoped that this study will pave way for routine high cycle fatigue tests of metals at the micron scale and provide clues for designing a similar indentation fatigue test.

In this work, we report the results from high strain rate impact tests on various materials using nanoindentation technology. A detailed description of the equipment and experiment is provided, including: (1) Dynamics of a single degree-of-freedom system; (2) Time constants on the load and displacement signals and their relationship to impact testing; (3) Calculations of displacement, velocity, and acceleration for a step force; and (4) Calculation of the impact loads using the stiffness, damping, and mass of the indenter. A diamond Berkovich indenter was used in the experiments and hardness, indentation strain rate, and the resulting strain rate sensitivity are reported for single crystal calcium fluoride, fused silica, polycarbonate, steel, and high-purity iron. Indentation strain rates on the order of 1x104 s-1 were observed during the experiments. The results of the impact testing reiterate the importance of careful characterization and understanding of the dynamic behavior of the instrument. Furthermore, it was found that the data acquisition rate must be sufficient to record enough data for the analysis as the typical time for the initial loading impact was on the order of 100 microseconds. Further details of the impact test are provided including just prior and just after the initial loading. The strain rates and strain rate sensitivity results are compared to existing uniaxial data and the advantages of using a nanoindenters to characterize the impact resistance of materials are highlighted. Lastly, limitations of the experiment, e.g., load frame resonance or calculation of unloading stiffness, and future improvements are discussed.

9:45 AM - TC06.11.05

Understanding the Effect of Interface In FCC-BCC Multilayered Nanocomposites through In Situ Microfracture Bending Experiments

Cu-Nb and Al-Nb multilayered nanocomposites, both FCC-BCC, exhibit different mechanical behaviour due to their varying interfacial shear strengths and properties. To understand this effect in their fracture behaviour, in situ microfracture testing inside a scanning electron microscope was conducted. Microscale beams of Cu-Nb and Al-Nb PVD nanolayers were loaded in the manner of the three-point bending test with fixed ends. Video capture was done to observe the crack behaviour while the experiment was performed. Cu-Nb nanolayers was found to have fractured along the grain boundaries without any crack initiation whereas Al-Nb nanolayers showed some difference in its deformation mechanism before a crack initiated at the notch and propagated straight through the beam. Such insights would not have been obtained in an ex situ experiment. Our in situ study provided unique insights on the microstructural origins of this crack propagation behaviour. This knowledge could thus lead to new fracture limits in nanomaterials design through interface engineering and hence open up design space for the use of advanced nanomaterials in general in real world systems.

10:00 AM -

BREAK

TC06.12: Fracture and Deformation

Session Chairs

Amine Benzerga

Daniel Kiener

Wednesday AM, November 29, 2017

Hynes, Level 2, Room 210

10:30 AM - *TC06.12.01

Deformation and Fracture Mechanisms of Nanostructured Materials at Elevated Temperatures

Understanding the mechanisms by which nanostructured materials deform and fail is of prime importance in order to improve their properties. This is in particular the case at elevated temperatures, where due to the high amount of interfaces in conjunction with the additional thermal activation a number of plasticity mechanisms can operate. These can potentially affecting the flow and failure behavior.Direct in-situ observation of the operating mechanisms can be achieved using scanning and transmission electron microscopes, which enables detailed insights and identification of the respective processes. Thereby, this strategy permits development of mechanism based plasticity and failure models for nanostructured materials at ambient and elevated temperatures.In this presentation, recent advances along these lines will be covered. We will start with assessing the local material flow behavior and rate sensitivities by spherical nanoindentation. Building on the stress-strain characteristics, the detailed temperature dependent deformation and failure mechanisms are assessed by in-situ compression and fracture testing in the scanning and transmission electron microscope.

Recent fracture experiments showed that introducing a circumferential notch in round-bars of a hot-rolled plate of AZ31 magnesium alloy can significantly increase the strain-to-fracture during in-plane tension. The presence of numerous deep dimples distinguishes the fracture surface of the notched bars from that of un-notched specimens, where facet-like features associated with twin-induced cracks are dominant. Based on post-mortem fracture surface observations, the higher strain-to-fracture in notch bars is partially attributed to a change in damage initiation mechanism from twin-induced cracking in uniaxial bars to void nucleation from second phase particles in notched specimens [1]. A lower propensity for shear localization and higher activity of extension twins and dislocations are also proposed to contribute to improved fracture properties of notched bars [1,2].

Here, we study the microstructure and state of damage in deformed AZ31 notched bars with two different geometries, using interrupted experiments along with high-resolution X-ray microtomography analyses. By utilizing notched bars, a controlled and more-or-less constant triaxiality is created in the notched region. In addition, the effects of stress state on the processes of damage accumulation and fracture of AZ31 can be studied via changes in the notch geometry. Macroscopic shear localization is also prevented in notched bars. Thus, a diffuse damage zone is introduced that facilitates the investigation of damage state without interference from shear localization.

The results corroborate the importance of second phase particles in the processes of damage and fracture in Mg alloys. Numerous voids/microcracks, originating from second phase particles, are found in the microstructure. Irrespective of their origin, voids/microcracks blunt to various degrees depending on factors such as their spatial distribution and local triaxiality and strain level. Despite blunting processes during plastic deformation, voids at failure are still oblate. The present study shows that acuity of the notch and its associated change in triaxiality affect the location and shape of macrocracks in notched Mg bars. At moderate triaxialities, multiple macrocracks initiate near the center of the notch and grow along the rolling direction. These cracks, then, link-up in the planes perpendicular to the rolling direction via shear bands and form a zig-zag pattern. At high triaxialities, macrocracks initiate near the free surface. Crack extension in this specimen proceeds without much blunting of the crack-like voids, leading to a seemingly brittle fracture path.

Nanoporous gold (np-Au) is a material with sponge-like structure, composed of continuous ligament and pore in nanoscale, which gives low density and high surface to volume ratio. From these advantages, there have been extensive researches to apply np-Au for catalyst, actuator, and sensor. However, brittle behavior of np-Au unlike ductile bulk gold remains as an issue to overcome. Previous researches have shown that brittleness of np-Au occurs by stress concentration on pore surface and catastrophic crack propagation through grain boundary, which had been formed at Au-Ag precursor alloy state. Here, we focus on the effect of the ligament/pore size, grain boundary density, and grain shape on fracture toughness of np-Au.We fabricate well-annealed, cold rolled, and hot rolled Au-Ag precursor alloy for microstructure variation. Well-annealed precursor alloy by melting Au and Ag has microscale grain. Cold rolled precursor alloy is nanocrystalline by recrystallization and hot rolled precursor alloy has anisotropic grain along the rolling direction. By free corrosion dealloying in nitric acid, Ag is selectively etched from alloy and np-Au is formed. Only well-annealed precursor alloy is annealed additionally for coarsening of np-Au, which increases ligament/pore size.Microstructure of precursor alloy and crack propagation are observed by SEM (scanning electron microscope) and EBSD (electron back-scattered diffraction). Stress intensity factor, KΙc is measured with three-point bending to observe the brittle behavior of np-Au, and we discuss effect of ligament/pore size, grain size and grain shape on fracture toughness of np-Au.

Low-temperature plastic deformation of bcc metals is governed by thermally activated glide of 1/2<111> screw dislocations. In general, the flow stress that is needed to initiate the plastic flow depends on the orientation and character of the applied load. Atomistic simulations on isolated screw dislocations made in the past decade have been indispensable to understand the governing mechanisms responsible for the initial stage of plastic flow in these materials. They have been represented by single-crystal yield and flow criteria for all bcc metals of the VB and VIB group. Here, we test the validity of these theoretical models by direct comparisons with slip trace analyses made on deformed high-purity single crystals of two non-magnetic bcc metals of the VB and VIB group at 77 K. For each metal, we consider both tension and compression along three different orientations that cover large area of the stereographic triangle. The slip traces are taken from two perpendicular sides of deformed samples on the surface area of approx. 1 mm2 using the laser focus sensor in Nanopositioning and Nanomeasuring Machine. The orientations of slip traces are correlated with the Schmid law as well as with the predictions of the yield criteria developed previously for these materials.

Nonlocal elastic constitutive laws are introduced for crystals containing defects such as dislocations and disclinations [1]. In addition to point wise elastic moduli tensors adequately reflecting the elastic response of defect-free regions by relating stresses to strains and couple-stresses to curvatures, elastic cross-moduli tensors relating strains to couple-stresses and curvatures to stresses within convolution integrals are derived from a nonlocal analysis of strains and curvatures in the defects cores. Sufficient conditions are derived for positive-definiteness of the resulting free energy, and stability of elastic solutions is ensured. The elastic stress/couple stress fields associated with prescribed dislocation/disclination density distributions and solving the momentum and moment of momentum balance equations in periodic media are determined by using a Fast Fourier Transform spectral method. The convoluted cross-moduli bring the following results: (i) Nonlocal stresses and couple stresses oppose their local counterparts in the defects core regions, playing the role of restoring forces and possibly ensuring spatiotemporal stability of the simulated defects, (ii) The couple stress fields are strongly affected by non-locality. Such effects favor the stability of the simulated grain boundaries and allow investigating their elastic interactions with extrinsic defects, (iii) Driving forces inducing grain growth or refinement derive from the self-stress and couple stress fields of grain boundaries in nanocrystalline configurations.

In this work Molecular dynamics simulations (MDs) and in-situ experimensts are used to understand the plastic deformation mechanisms of single crystal silicon (SCS) at high temperatures. Here, we present a novel silicon MEMS stage for in situ bending test of micro/nanoscale samples at high temperature. The new stage minimizes uniaxial state of stress in the specimen, but maximizes the bending stress over a small volume such that high stresses can be reached within a small volume on the specimen without causing a premature failure by fracture. A test setup was designed to fit inside an SEM and be able to heat the sample up to 500C while applying sufficient bending stress by stretching the stage. The chip-based stage, actuated by a piezo-actuator, applies bending moments on micro/nanoscale beam specimens. Analytical and finite element (FE) models were developed to predict the behavior of the bending stage and calculate the stresses at the anchors. SCS micro-beams oriented along [0 1 1] are tested at room temperature under bending, and showed high strength compared to the uniaxial tension test results. The calculated modulus of elasticity resulting from experimental data matches the value reported in literature. Specimens with thicknesses 2-5 µm exhibited brittle to ductile transition (BDT) at 400C. These specimens show a significant reduction in BDT temperature compared to their bulk counterparts (550C). Dislocations and nanotwins were studied using TEM imaging. MDs are implemented to calculate the lattice parameter at higher temperature, and reveal the plastic deformation mechanism under bending. Four slip systems {1 1 1} <1 1 0> are activated in both simulations and experiments.

1:45 PM - TC06.13.02

Brittle to Ductile Transition in Silicon Nanopillars—Confrontation of Simulations and Experiments

While bulk silicon is brittle at temperatures below 600-700K, the compression of nanopillars has shown that a decrease of the diameter below few hundreds of nanometers could change the silicon behavior from brittle to ductile [1]. As silicon is free of initial defect, this size effect should be controlled by the cracks and/or the dislocations nucleation from the surface. The identification of the parameters governing the brittle to ductile transition in size and the understanding of the mechanisms are the key points to prevent the failure of microelectronic components based on the silicon strained technology. Nowadays the respective improvements in simulations and experiments allow to investigate the mechanical properties of objects of similar sizes, close to hundreds of nanometers. We considered atomistic simulations based on two different semi-empirical potentials fitted to better reproduce the ductile and brittle properties of bulk silicon. The simulations under tensile load show the nucleation of perfect dislocations from the surface that can lead to cavity opening when they interact [2]. Second, we show that the brittle to ductile transition is not abrupt and mainly controlled by the diameter of the nanopillars, as observed experimentally in compression. The underlying mechanisms will be detailed, and a criterion based on the quantification of the plastic deformation before cracks opening will be presented to measure the degree of ductility of the breaking. The simulations will be confronted to the results of the experimental deformations performed on nanopillars with diameter of 100nm.

Dislocation nucleation in silicon has been the topic of intense research because it is essential to the understanding of plastic deformation, ductility and mechanical strength of semiconductor devices. Out atomistic models predict that a glissile dislocation complex nucleates from the (001) surface of silicon films under a compressive strain much smaller than all other types of glissile dislocations considered previously. This dislocation complex consists of an intrinsic stacking fault on the glide- set (111) plane and two partial dislocations on the neighboring shuffle-set planes, contrary to the common notion that partial dislocations cannot exist on shuffle-set planes. This dislocation complex reduces the long-standing gap between experimental observations and atomistic models in terms of the critical strain needed to nucleate a dislocation in silicon-based semiconductors.

In this work we present the results of molecular dynamics simulations of the uniaxial compression of silicon nanoparticles using a modified Stillinger-Weber potential. The aim of the study is to investigate the first stages of plasticity while focusing on the effects of their shape and size. Therefore, we have prepared nanoparticles having sizes between 10-50nm with spherical, quasi-spherical, facetted and cubic geometries, which were afterwards deformed at a constant velocity, by placing indenters parallel to the (100) and (111) surfaces. We investigate the effect of the nanoparticle shape on the elastic stress field, on possible phase transitions and on the defects that are formed at the onset of plasticity. We also discuss the nucleation sites and characteristics of the dislocations that are formed in the plastic phase, as well as their activity during loading and their dependence on temperature. Finally, we comment on the effect of the structure on the yield stress and draw analogies to previous experimental studies.

The deformation behavior of metallic single crystals is size dependent, as shown by several studies during the last decade. Nevertheless, real structures exhibit different interfaces like grain, twin or phase boundaries. Due to the possibly higher stresses at the micron scale, the poor availability of dislocation sources and the importance of diffusion in small dimensions the mechanical behavior of samples containing interfaces can considerable differ from bulk materials. Within this study we will show the size scaling behavior of general high angle grain boundaries in copper. The boundary presented is believed to show extensive dislocation slip transmission at bulk dimensions.In the talk results from in situ scanning electron microscopy (SEM) and in situ µLaue diffraction will be shown. While the SEM data is used to proof slip transmission, µLaue is probing the occurrence of dislocation pile-ups at the grain boundary. The results show that at low plastic strains and strain rates the size scaling behavior of single and bi-crystalline samples is identical in cases where the grain size is assumed as the critical length scale. It can therefore be concluded that the initial number and size of dislocation sources is dominating not only the deformation behavior of single crystalline pillars, but also for bi-crystals (at low plastic strains). Thus, the character of the boundary does not play any role for the mechanical properties at the onset of yield during slow deformation. This behavior is vastly different when higher strain rates up to 10-1s-1 are applied. Then, the bi-crystalline sample shows significantly higher yield stresses.While at low strains and strain rates the initial source size is key for the deformation behavior, higher strains and strain rates hint towards the non-conservative motion of dislocations in the grain boundary plane as the limiting process. In the talk, this will discussed together with possible strategies to implement the experimental findings to discrete dislocation dynamics simulations.

4:00 PM - TC06.14.02/TC02.10.02

Use of In Situ TEM to Characterize the Deformation-Induced Martensitic Transformation in 304 Stainless Steel

Djamel Kaoumi 1 1 , North Carolina State University, Raleigh, North Carolina, United States

Tensile tests are conducted in-situ in a TEM at room temperature down to cryogenic temperatures (from -100°C to 0°C) using a cooling TEM straining-stage with the goal of capturing the growth of the martensitic phase as it develops under stress in the material. The formation of stacking faults was captured, as well as the subsequent formation of ε-martensite, confirming the role played by SFs as intermediate step during the transformation from γ-austenite to ε-martensite. In addition, direct transformation from γ-austenite to α’-martensite was captured as well (i) upon straining at a fixed temperature and (ii) upon cooling after pulling on the sample, indicating again how stress and temperature are both effective on the transformation.

Modern cold-forming processes subject metals to multiaxial stress states and strain path changes which are not accurately modeled using uniaxial mechanical testing. It is therefore important to examine the material behavior during the complex strain paths experienced in industry. Here, in situ diffraction techniques are used to study the influence of non-linear strain path changes on the deformation mechanisms, microstructure evolution and residual stress development of a cold-rolled Mg AZ31B with strong basal texture. Neutron powder diffraction with acoustic emission and Laue microdiffraction are used to link the bulk macroscale behavior of the alloy with specific deformation mechanisms observed at the microscale, in a single grain.The deformation behavior of the AZ31B was examined during three load path changes of 45, 55, and 90 degrees from a uniaxial preload using combined in situ neutron powder diffraction (POLDI beamline, SINQ) and acoustic emission. All loads were in-plane tension. The results show that the active strain accommodation mechanisms ({10-12}<10-11> extension twinning vs. basal slip) in the second load depend strongly on the angle of strain path change. Laue microdiffraction experiments were then performed (MicroXAS beamline, SLS) to examine active mechanisms within a single grain during bulk deformation of the polycrystal. The results are used to link the microscale behavior with the bulk properties observed in the neutron experiments, and more closely examine the role of strain path changes on twin variant selection. Additionally, EBSD and TEM images from various stages of deformation have also been prepared; the microstructure changes are examined with respect to the lattice strain evolution measured during the in situ tests.This research is performed within the ERC Advanced Grant MULTIAX (339245).

The ultrahigh strain rate behavior of lightweight energy-absorbing materials, such as glassy-rubbery block copolymers: polystyrene - polydimethyl siloxane (PS-PDMS), thin multilayer graphene films, polymer grafted nanoparticle films and single crystal silver microcubes, is explored using a miniaturized ballistic test: LIPIT, Laser Induced Projectile Impact Test. Micron sized projectiles are launched at various targets using a laser pulse, and the deformation field around the embedded projectile is analyzed for thick targets, while penetration occurs for thin targets and projectile deformation occurs in the case of a soft projectile/hard target. Such studies provide valuable information on how materials respond to very large strains at very high (~ 107 s-1) strain rates, important for applications such as advanced protective materials. The small size of the projectiles provides a deformation field that is < tens of microns in size and can be examined at high resolution. Characterization is done by a combination of dual SEM and ion beam microscopy as well as TEM of thin slices. Correlation of these micro scale tests to models as well as how the behavior scales to the macroscale are being explored.

X-ray topography is a well-established method to visualize dislocations and associated strain fields in single crystals. It is based on Bragg diffraction and provides a two-dimensional intensity mapping of the diffracted beam. The resolution is typically limited by the pixel size of the detector, which restricts its practical usage to relatively large objects and defects. In this work we present a new method, ptychographic topography, where we combine tele-ptychography and Bragg topography. In tele-ptychography an object is illuminated with a parallel x-ray beam and the wave front after propagation through the object is reconstructed using an analyzer downstream the sample. In combination with Bragg topography it allows obtaining high-resolution topographs and in contrast to conventional topography it additionally provides phase contrast.We apply this method to visualize the strain field around an indent in a thin Si wafer and in a compression Cu micropillar. Here we combine ptychographic topography with rocking curve imaging. This involves recording high-resolution topographs while rocking through a Bragg peak. Each pixel of the image records it own local rocking curve that is analysed in terms of integrated intensity, angular position and width.Ptychographic topography has the advantage that the sample is not scanned during ptychographic acquisitions, which renders the method compatible with in situ experiments. Furthermore, the technique relies on the use of a parallel beam, which ensures that, in contrast to Bragg projection ptychography, the illumination remains approximately constant during a rocking curve scan.

TC06.15: Poster Session III: Confined Volumes III

Session Chairs

Dan Mordehai

Wednesday PM, November 29, 2017

Hynes, Level 1, Hall B

8:00 PM - TC06.15.01

The Effect of Stress/Strain Gradient and Grain Boundaries on Mechanics of Metals

The observable properties of solids and structures are only the macroscopic manifestation of very many complex interaction mechanisms involving structural components on numerous scales smaller than those we perceive with the naked eye. The material's properties of interest comprise, its mechanical performance like strength and ductility. All of the macroscopic material characteristics stem from the various interaction mechanisms that occur across various length scales. Therefore, it is of enormous importance to develop techniques that explore the multiscale nature of materials in order to accurately describe, to fundamentally understand, and to reliably predict the mechanics and physics of solids.We employ a multi-scale framework that couples the viscoplasticty (VPSC) model developed by Lebenson and Tome and an in house developed continuum dislocation density (CDD) model to simulate dislocations/grain boundaries interaction in polycrystalline metals. We describe the plastic deformation in terms of the plastic slip on active slip systems of the crystal, from which one can obtain Nye's dislocation density tensor that contains information about the dislocation density at a material point. In this work, the effect of stress and strain gradient and grain boundary of several nano-microstructures is analyzed by the mentioned modeling tools. Generally, where there are strain gradients, there are stress gradients as well. Models based on stress-gradient and strain-gradient theories can predict size effects unlike the conventional models of crystal plasticity. Size effect in the stress-gradient plasticity theory originates from dislocation pile-ups against grain boundaries and or obstacles under inhomogeneous stress state. Thus, pile up length in each grain will be considered as material length scale that affect hardening. Furthermore, the interaction of mobile dislocations with grain boundaries is of essential interest in this research with a particular focus on magnesium. Depending on the misorientation of neighboring grains and grain size, each GB can have different strengthening behavior. A dislocation density based model is introduced to analyze slip transmission across grain boundaries in polycrystalline Mg. The model takes full account of the geometry of grain boundary, normal and direction of incoming and outgoing slip systems, as well as the extended stress field of the boundary and dislocation pileups at the boundary. The role of stress/strain gradient and grain boundary on strength and ductility can expand our knowledge of mechanical behavior of metallic nano microstructure Mg. Also experimental validation is performed in the following ways: 1) EBSD to map grain size and intergranular misorientation, 2) Tensile test to obtain stress-strain curve, 3) Nano-indentation to verify the role of grain boundary on dislocation flux.

Nanolattice as fabricated by two-photon lithography (TPL) is a coupling of size-dependent material properties at nanoscale with structural geometry response in wide application of micro/nano manufacturing. Yet to date, the quantitive mechanical analysis on the nanolattice at micro scale is still rarely reported. In our work, 3-dimensional (3D) polymeric nanolattices were initially fabricated using TPL. The polymer structures were then conformably coated with a thin 80 nm high entropy alloys (HEAs) film (CoCrFeNiAl0.3) via physical vapor deposition (PVD). To our best knowledge, HEA is proposed for the first time to fabricate the composite nanolattices combined with 3D laser lithography. Additionally, the mechanical properties of the obtained composite architecture were investigated via in situ scanning electron microscope (SEM) compression test and the presented architecture encouragingly exhibits ultrahigh compressive specific strength of 0.032 MPa kg-1 m-3 with density below 1,000 kg m-3.

8:00 PM - TC06.15.04

Coupled Thermo-Mechanical Gradient Dependent Plasticity Formulation Based on Small and Large Deformation Theories

Yooseob Song 1 , George Voyiadjis 1 1 Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana, United States

In this work, a coupled thermo-mechanical gradient enhanced continuum plasticity formulation is developed within the thermodynamically consistent framework and corresponding two-dimensional finite element analysis is implemented to examine the micro-mechanical and thermal characteristics of the small-scale metallic volumes based on the small and large deformation theories. In the first part of the work, the theory based on the small deformation is proposed with the concept of thermal activation energy and the dislocations interaction mechanisms. The theory is also based on the decomposition of the thermodynamic microforces into energetic and dissipative counterparts, decomposition of free energy into the elastic, defect and thermal counterparts, and decomposition of dissipation potential into the mechanical and thermal counterparts. The temperature distribution in the system, due to the conversion of the plastic work into the heat and the partial dissipation of the heat due to the fast transient time, is included into the model using a generalized heat equation. The derived constitutive model is validated through the comparison with the experimental observations conducted on micro-scale thin films. The proposed model is applied to the simple shear problem and the square plate problem respectively in order to investigate the thermo-mechanical behavior and the grain boundary effect of small-scale metallic materials. In the second part of the work, two-dimensional finite element simulation for the finite deformation incorporating the temperature effect is developed based on the implicit gradient-enhanced approach, F-bar method and radial return algorithm. The proposed model is applied to shear band problem to examine the mesh sensitivity issue. In classical plasticity theory, the numerical solutions of the localization problems for the rate-independent solids exhibit the inherent mesh dependence, and the minimum width of the band of localized deformation is set by the mesh density. In this work, the material intrinsic length scale, which is related to the microstructure and the failure mechanisms during plastic slip, to the classical plasticity theory is introduced in order to overcome this mesh sensitivity problem. Finally, some remarkable conclusions are addressed.

Dynamic Mechanical Analysis (DMA) is an indispensable tool for determining the viscoelastic properties of polymer composites in order to support product development and quality control. By 5-dimensional system stiffness calibration (sample holder, temperature, dynamic force, amplitude range, frequency) highest E’ and lowest tan δ can be measured, which is good for composites with high stiffness. With standard DMA deformation modes, information on oriental behavior, multifrequency effect, cure and master curve can be investigated. In contrast to 3-point bending for standard materials such as polymers, single cantilever bending with free push-rod is designed for very stiff materials (ideal for Young‘s Modulus approx. >30.000 MPa, such as carbon fiber-reinforced plastic (CFRP)). Many accessories are tailored for different special applications such as simultaneous DMA-DEA, UV-DMA, and immersion and humidity tests for aging and fatigue studies (plasticizer effect of oil or water).DMA EPLEXOR® series with high force capabilities up to +/-8000 N are ideal for nonlinear properties and fatigue tests (hysteresis) on polymer composites. For composites with special application, measurement can be made in tension/compression mode, even at high temperature up to 1500°C.

8:00 PM - TC06.15.06

The Effect of Moisture on Anisotropic Elastic Moduli of Lignocellulosic Cell Walls

A combination of experimental, theoretical and numerical studies is used to investigate the variation of elastic moduli of lignocellulosic cell walls of bamboo fiber with moisture content (MC). Our Nanoindentation results show that the longitudinal elastic modulus initially increased to a maximum value at about 3% MC and then decreased linearly with increasing MC. We showed that this behavior is influenced mostly by the amorphous materials in cell walls. Variation of Elastic modulus of lignin and hemicellulose, calculated by molecular dynamics simulations, indicates that lignin has a key role in the variation of overall elastic modulus of cell wall with increasing MC. Below 10% MC, during a bridging process between polymer chains by water molecules, the fractional free volume of lignin decreases. The free volume reduction along with formation of hydrogen bond bridges causes a growth in elastic modulus of cell wall at low MC.

8:00 PM - TC06.15.07

Unraveling the Thermomechanical Behavior of Semiconducting Polymers Using Computer Simulations and Experiments

A detailed understanding of the mechanical properties of semiconducting polymers is of critical importance for roll-to- roll production and thermomechanical reliability of organic electronic devices. These material properties are a complex function of many factors—including molecular structure, composition, and processing conditions—making them difficult to design by chemical intuition and empiricism alone. Here, we apply both classical atomistic and coarse-grained molecular dynamics simulations to predict the solid-state morphology, thermal, and tensile behavior of a representative set conjugated polymers and composites (including pure P3HT, several low bandgap polymers, and composites with fullerene derivatives). The approach is first validated through comparison to experiment for important thermomechanical properties including density, tensile modulus, and glass transition temperature. Detailed analysis uncovers the underlying molecular mechanisms for strain accommodation in this class of materials including the role of solid-state packing, chain conformations, and entanglement on bulk mechanical properties. Additionally, a novel experimental technique is introduced for measuring the glass transition temperature in thin films (~100 nm) of semiconducting polymers using UV-Vis spectroscopy.

8:00 PM - TC06.15.08

Improving the Performance of Pressure Sensitive Adhesives by Tuning Cross-Linking Locations and Density

Pressure sensitive adhesives (PSA) are a class of adhesives that flow under pressure for wetting the adherend and simultaneously resist flow for carrying loads. A comprehensive study of how their overall mechanical properties are related to the molecular-scale structure is a key for improving their performance. Cross-linking is an effective way of tuning the mechanical properties of polymers. In the present work, we developed a coarse-grained model of PSA for assessing the effect of cross-linking densities and locations on the performance in tack experiments of PSAs with different polydispersivities. It was found that placing cross-linking sites at the ends of polymer chains showed advantages over random cross-linking locations and cross-linking locations concentrated at the middle part of polymer chains. The PSA with cross-links at the ends of polymer chains showed higher toughness and lower cross-linking density for the transition of failure mode from cohesive failure (internal failure of the adhesive material) to adhesive failure (debonding of the adhesive from the surface without leaving a residue). A detailed examination of the connectivity and morphology of the polymer networks showed that the end cross-linked networks were more extensive and had more space for sliding between polymer chains, which is the main contributor to energy dissipation. Our simulation results provide strategies for designing high-performance, highly stretchable PSAs.

8:00 PM - TC06.15.09

Effects of Notches on the Deformation Behavior of Nanoscale Metallic Glasses

Reducing the size of metallic glasses (MG) to submicron and nanoscale levels improves their strength and ductility. However, there is no clear consensus in the literature regarding their mechanical behavior in the presence of a flaw or notch. Quantitative tensile tests on notched nanoscale CuZr MG specimens were conducted inside a transmission electron microscope to study their deformation characteristics. Strength was found to be notch insensitive for shallow notched thick specimens, although reducing specimen dimensions and increasing notch sharpness enhances it by 14%. It was reasoned that the severity of geometrically constraining the growth of shear bands dictates the strength and fracture morphology of notched specimens. Softening, accompanied with a transition to necking failure occurs when the unnotched ligament width < 80 nm. The competition between embryonic shear band growth and free volume mediated homogeneous activation of STZs was found to be responsible for this brittle to ductile transition (BDT). Current results provide unique insights into the various design aspects to be considered for reliable engineering of small scale components.

The exceptional mechanical, electronic, and thermal properties of graphene render it a highly promising filler for polymer-matrix nanocomposites. While preliminary experiments show significant enhancement in the thermomechanical properties of graphene-polymer nanocomposites compared to those of the polymer matrix even at relatively low graphene loading, our fundamental understanding of structure-property relationships that govern such enhanced materials response remains very limited. Toward this end, in this presentation, we report a multiscale atomistic-to-continuum approach for modeling interfacial stress transfer in graphene-reinforced high-density polyethylene (HDPE) nanocomposites. Via detailed characterization of atomic-level stress profiles in sub-micron graphene fillers, we develop a modified shear-lag model for short fillers. A key feature of our approach lies in the correct accounting of stress concentration at the ends of fillers that exhibits a power-law dependence on filler (“flaw”) size, determined explicitly from atomistic simulations, without any ad hoc modeling assumptions. In addition to two parameters that quantify the end-stress concentration, only one additional shear-lag parameter is required to quantify the atomic-level stress profiles in graphene fillers. This three-parameter model is found to be reliable for fillers with dimensions as small as 10 nm. Our model predicts accurately the elastic response of aligned graphene-HDPE composites and provides appropriate upper bounds for the elastic moduli of nanocomposites with more realistic randomly distributed and oriented fillers. This work provides a systematic approach for developing hierarchical multiscale models of 2D material-based nanocomposites and is of particular relevance for short fillers, which are currently typical of solution-processed 2D materials.

8:00 PM - TC06.15.11

In Situ Micro-Ballistic Characterization of Carbon Nanotube Fibers in Comparison with Kevlar and Nylon Fiber Filaments

Armor performance of ballistic fabrics is primarily dependent on the intrinsic characteristics of the individual fibers of the fabrics. Because carbon nanotubes (CNT) have displayed high strength and stiffness combined with low density, they are very promising, with much higher theoretical tensile strength over polymeric fibers. Up to now, mechanical tests on CNT fibers have mostly been within the quasi-static regime, which is inadequate for assessing the armor performance of CNT fibers. For the first time, we demonstrate supersonic impacts of a micro-projectile on a CNT fiber. Three other fibers, Kevlar KM2 filaments, Nylon 6,6 filaments, and pure aluminum filaments, are also investigated under the same conditions for comparison. The fiber (~10 um in dia.) is mounted in air using two epoxy supports and individual glass spheres (~ 30 um in dia.) impact on the fiber perpendicular to the fiber’s axial direction at ~500 m/s. The real-time deformation process is recorded using an ultrafast microscopic imaging system for accurate kinetic information of the impacting micro-projectiles and the responding fiber. We observe the characteristic V-shape deformation of each fiber during the impact, and measure instantaneous velocity and acceleration of the projectile. In terms of the specific energy dissipation rate of the micro-projectile interacting with the fiber, the CNT fibers demonstrate superior armor performance to the other three fibers, including Kevlar KM2, primarily due to the highest transverse wave speed of the CNT fibers. Scanning electron microscopy (SEM) is used to study the post-impact damage features of the fibers. For the CNT fibers, Raman spectroscopy mapping is used to reveal the impact-induced lattice damage of CNTs.

Metallic glass matrix composites (MGMCs) possess superior mechanical properties which can sustain notable plasticity under high strength. Such unique properties may result from delocalizing, blunting, or branching behaviors of shear bands connecting with extended defects in the crystalline second phase. Fundamental research to understand and control the dynamics of flow defects in MGMCs is key for them to serve as advanced structural materials and to drive their commercialization successfully. Compared to the improvement of strength and ductility influenced by dislocation models in crystalline second phase, the improvement induced by twins receives less attention in constitutive modeling and simulations. A mesoscale model combining the kinetic Monte Carlo algorithm and the finite element method is developed to investigate how ductile inclusions affect the initiation and the interaction of shear bands and twins. This model can extend simulation times and sizes for the study of plastic deformation in MGMCs up to scales of those in laboratory experiments, which cannot be accessed by classical molecular dynamics. Shear transformation zone (STZ) dynamics proposed in previous work [P.Y. Zhao, J. Li, and Y.Z. Wang, Int. J. Plasticity 40, 1 (2013)] renders the behavior of the amorphous matrix. Dynamics of twins dominating the behavior of the inclusions is constructed in a similar way to STZ dynamics. This work attempts to examine the effects of microstructural factors such as volume fraction, inclusion size and shape on the MGMCs behavior in order to inspire new strategies for the design of better MGMCs based on the defect engineering concept.

Over the past decades, the notion of localized rearrangements called shear transformations has been used to model the plastic deformation of a wide range of amorphous solids such as bulk metallic glasses, emulsions, pastes or granular materials. A variety of mesoscopic models have been built on the picture of interacting shear transformations, hence the mesoscale description of amorphous plasticity results from the interplay between elasticity and disorder. These coarse-grained models have been proven successful in reproducing shear localization and various critical properties of amorphous systems under deformation, such as avalanche exponents, strain correlations or Herschel-Bulkley exponents. One of the most important discrepancies however stems from the finite-size scaling of the diffusion coefficient: Martens et al. found that the diffusion coefficient D increases with the system size as D~L^1.5, while several independent zero-temperature molecular dynamics simulation studies have found D~L or D~log L. Here we resolve this discrepancy for small deformations and show that D~L can be recovered up to a strain window necessary for a single shear band to form. D~L gradually transforms into D~L^1.5 as multiple shear bands form showing that in these models subsequent shear bands are not statistically independent. We show that special care has to be taken when building the elastic interaction: confirming previous results, we show that the lack of soft deformation modes in the elastic interaction leads to the breakdown of diffusion at long times.

Vertically aligned helical carbon nanotube (HCNT) forests have been studied due to their interesting properties for electromagnetic shielding [1], energy storage [2], and impact protection [3]. Formed by entangled helical coils of multiwalled carbon nanotubes, HCNT forests exhibit behaviors fundamentally different from vertically aligned straight carbon nanotube forests such as nonlinear force-displacement behavior [3-5] and super-compressibility, being able to recover up to 80% of compressive strains [4]. These behaviors are partially attributed to the intrinsic geometric nonlinearity of the individual HCNTs and to the entangled morphology present in HCNT forests. Understanding the relative contribution of those effects on the compressive responses of HCNT forests is crucial to improve their ability to mitigate impact. In this work, we developed a coarse-grained, mesoscale model that includes intra- and inter-HCNTs interactions to study the mechanical compressive response of HCNT forests. The forests were modeled by a 10x10 regular array of identical HCNTs with periodic boundary conditions and with constant density along the forest. Each HCNT was comprised of beads regularly distributed along an helix. Bead interactions were described through harmonic stretching and bending terms for bonded interactions, and through van der Waals terms for non-bonded interactions. Compressive tests up to 0.5 of strain were performed for forests with different initial separations s (in terms of coil diameter D) between neighboring HCNTs to simulate different initial entanglements (i.e. overlap among coils). The values for the compression velocity, coil diameter and pitch were obtained from [4]. Stress(σ)/strain(ε) curves and entanglement evolution were obtained from classical molecular dynamics simulations. For larger initial entanglements (s<D), larger compressive forces, small coil bending, no buckling, no crushing, and a monotonically increasing entanglement (from 50 to 70% of variation) were observed. On the other hand, for a smaller initial entanglement (s=D), coil bending, buckling, and crushing were observed mainly on the top of the forest. In addition, the entanglement behavior exhibited an initial decreasing followed by increasing with 25% of variation. For those cases, stress-strain curves comprised of an initial linear regime (σ~ε) followed by a nonlinear regime (σ~ln[(0.5-εc)/(0.5-ε)]). This behavior is similar to the one observed for super-compressible foamlike vertically aligned carbon nanotube forests when waviness, produced by repeated compression, is present [5]. The transition between these regimes depends on s and occurs at the critical strain εc.References:[1] S. H. Park, et al. Appl. Phys. Lett. 96 043115 (2010)[2] A. L. M. Reddy, et al. J. Mater. Chem. 21 16103 (2011)[3] C. Daraio, et al. J. Appl. Phys. 100 064309 (2006)[4] R. Thevamaran, et al. RSC Adv. 5 29306 (2015)[5] A. Cao, et al. Science 310 1307 (2005)

8:00 PM - TC06.15.15

Effect of a Surface Constraining Layer on the Plastic Deformation of Au Microspheres

Microspherical particles are currently being used in applications as diverse as bio-medical drug delivery systems to large surface area electrodes energy-storage devices. With the increased application of metal microspherical particles it is necessary to study the effect of extrinsic constraint on their deformation mechanisms. In this study, single crystal Au microspheres of 3 μm diameter were coated with a sputter-deposited nano-crystalline Ni layer of 40 or 80 nm thickness. Room temperature compression tests were performed at three loading rates. SEM images of the deformed microspheres displayed micro-cracking of the deposited Ni layer during plastic deformation. Force-displacement (F-h) curves of the coated Au microspheres were obtained and compared with F-h curves from similar diameter uncoated Au spheres. The initial portion of the F-h curves was fitted with a Hertzian contact model and the corresponding incipient force was measured. The estimated apparent activation volume and energy corresponding to the initiation of incipient plastic deformation was found to be nearly identical for the coated and the uncoated Au microspheres. This suggests that the mechanism responsible for the initiation of first dislocation motion in these spheres is essentially the same regardless of the presence of a constraining coating. The apparent activation volume and energy of the rate-dependent deformation process after the Au microspheres have endured significant (about 5%) plastic strain is increased for the coated spheres compared to the uncoated spheres and increases with increasing Ni layer thickness. This reflects the effect of the Ni layer in constraining the motion of mobile dislocations and preventing them from reaching the free surface of the microspheres.

Liquid crystalline phases found in many biological materials, such as actin, DNA, cellulose, and collagen can be responsible for the deformation of cell membranes. In this work, cell membrane deformation is investigated through the coupling between liquid crystal anisotropy and membrane bending elasticity. The generalized shape equation for anisotropic interfaces, which resort to the Cahn-Hoffman capillarity vector, the Rapini-Papoular anchoring energy, and the Helfrich elastic energy, is applied to gain insight into the deformation of closed liquid crystal membranes. This study presents a general morphological phase diagram of membrane micro/nano-structured surface patterns, in which two characteristic regimes of membrane shapes can be classified with respect to the most dominant factor between liquid crystal anisotropy and bending elasticity. To that end, we consider a 2D nematic liquid crystal droplet immersed in a isotropic phase in the presence of an interfacial layer of surfactants, which leads to an additional elastic contribution to the free energy of the system. The presented results indicate that, depending on the bending elasticity of the cell membrane, the liquid crystal might be able to deform the cell, thereby resulting in anisotropic asymmetric shapes. As liquid crystal anisotropy dominates the bending elasticity, spindle-like or tactoid shapes, which are extensively observed in experiments, can be formed. The findings provide a foundational framework to better understand membrane topologies in living soft matters. Furthermore, the coupling between order and curvature of membranes shed new light into the design of novel functional soft materials.

8:00 PM - TC06.15.17

Local Mechanical Studies of Soft Materials with Nanoindentor and Atomic Force Microscopy

Quantitative studies of mechanical properties are often examined with Nanoindentor (NI) that is applied to measure a sample deformation (h) under different loads (P). Similar dependence can be recorded in Atomic Force Microscopy (AFM) when an ultra-sharp is pressed into a sample surface. These studies can be performed at separate locations or during profiling sample topography in non-resonant oscillatory Quick Sense mode. In all cases the h-P dependence is analyzed using solid state deformation models to extract elastic modulus, hardness, work of adhesion. These techniques are essentially complementing each other. NI is characterized by high precision of calibrated applied force (above 10 nN) and sample deformations measured with nm sensitivity. In AFM experiment the probe forces and deformations are much smaller. The latter features are important for nanoscale studies of soft materials. In practical evaluation of NI and AFM of such materials we have examined several polymers with macroscopic elastic moduli in the 1 MPa – 10 GPa range and soft metals with elastic modulus of tens of GPa. The measurements were conducted at ambient conditions using Keysight 9500 scanning probe microscope and Nanoindentor with applied forces from several nanoNewtons (nN) to several microNewtons (mN). The analysis of h-P data obtained with both techniques was performed in framework of several theoretical models: Hertz, DMT, JKR, O-P. The advantages and limitations of these models are considered in their applications to a number of polymers with pure conservative and dissipative mechanical response. The results, which were obtained with NI and AFM on samples of neat amorphous polymers at mN forces, showed similar values of elastic modulus. A complicated situation was observed in studies of semicrystalline polymers and polymer blends where mechanical properties vary at the nanometer scale. A rational approach to nanomechanical analysis of such compounds will be suggested. The applications of NI and AFM to examination of time-dependent mechanical properties of polymers will be also discussed.

8:00 PM - TC06.15.18

Effect of Processing on Indentation Results Obtained for High Purity Iron Sample

The role of sample processing on nano-indentation results at the grain boundary and grain interior is examined for high purity α-Fe. Indentation of surfaces with unresolved surface deformation due to mechanical polishing are compared to the same sample surfaces annealed at high temperature in a reducing atmosphere. Behaviors of interest include discontinuities in P-h curves at grain boundaries vs grain interiors and any variance in the indentation size effect. The differences in indentation behavior with annealing time and temperature are correlated with EBSD as an independent measure of surface layer crystallinity. The results are rationalized with a kinetic model for dislocation climb driven by surface image forces. The rate limiting step is considered to be the climb of high aspect ratio dislocation semi-loops intersecting the free surface. The growth of a Rayleigh type shape instability of the dislocation loop, driven by the surface image stress, is suggested as a mechanism to explain the dislocation climb kinetics and the observed reduction in surface hardness from the annealing process.This work was supported by Office of Basic Energy Sciences within the Department of Energy (DOE) Office of Science: Award Number: DE-SC0016314

8:00 PM - TC06.15.19

Nanoindentation in Discrete Dislocation Dynamics in Two- and Three-Dimensions—Probing Plasticity and Dislocation Structure

We present an investigation of the indentation size effects in crystals at the nanoscale, through extensive two and three dimensional discrete dislocation dynamics simulations. We focus on statistical features of nanoindentation and stochastic components, especially in relation to the effect of loading modes (displacement control versus load control). We demonstrate that size effects always exists, but different loading modes and different initial dislocation microstructure influence the hardness value and pop-in statistics. Nanoindentation can be a microscopic probe of plasticity, and we demonstrate that through studying the effect of pre-stress on nanoindentation hardness: we have found that at small indentation depth, hardness decreases with increasing tensile pre-stress; while at larger indentation depth, effect disappears.

8:00 PM - TC06.15.20

Effect of Crack-Tip Stress Fields on the Behavior of Phase Transition in Two-Phase Systems—A Phase Field Study

We present a quantitative phase field model for investigating the effect of crack-tip stress fields on the behavior of phase transition in two-phase systems. The model takes into consideration the effect of fracture mode, misfit strain, and elastic heterogeneity on the different stages of phase transition, e.g. phase separation/nucleation, growth, and coarsening. The phase field and stress equilibrium equations are solved using a fully-coupled, fully-implicit scheme implemented via the finite-element framework MOOSE. The model predicts a strong effect of fracture mode and misfit strain on all stages of phase transition. Preferential phase separation and coarsening are observed either in front of the crack-tip or at the back of the crack-tip depending on the combination of the misfit strain and fracture mode. Therefore, the kinetics of crack propagation and growth in these systems can be solely dominated by the mechanical properties of either the matrix/parent phase or the precipitate phase.

Epoxy is widely used as structural adhesive for bonded material systems in aerospace, construction, microelectronics and other industrial applications. In order to achieve better performance, carbon nanotubes are often applied as reinforced additives due to the extraordinary mechanical properties. The resulting carbon nanotube-reinforced epoxy has presented enhancement in the bonding strength and durability during long-term sustained loading. However, the bonded material systems in reality usually suffer from different ambient environments, especially varying temperature conditions. The evaluation of temperature effect on creep responses at the interface between epoxy adhesive and substrate becomes an essential issue. The investigation is conducted using molecular dynamics simulations to study the interfacial creep behavior in the bilayer system containing carbon nanotube-reinforced epoxy and silica substrate. The simulation results show the atomic motion at the interface region under constant shear loading at various temperature levels, and indicate the improved properties with the addition of carbon nanotubes in epoxy. The study enriches the understanding of temperature effect on the interfacial creep behavior at the atomic level, and provides promising predictions and guidelines for the design of composite materials in long-term applications.

8:00 PM - TC06.15.22

A Simple Implementation Method for Predicting of the Elastic Properties for Periodic Composite Materials

Wenkai Qi 1 1 College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu, China

Asymptotic homogenization (AH) method is a powerfulmathematically rigorous technique but not easily to be implemented for analyzing effective properties of periodic composite materials.In this paper, an improved implementation technique of AH is developed with the aid of commercial FEM software as a tool box. Then, abundant structural elements (like beam, shell and solid elements) in commercial software can be used to model unit cell with complex substructures, while simultaneously reducing the model to a small scale with less amount of calculation. During the implementation, a set of simple displacement boundary conditions are assumed for unit cells, and final effective elastic constant can be directly calculated after several static analysis. Three representative examples of applications are chosen and discussed to verify the validity and applicability of the new implementation method by comparing with other methods. The proposed method is expected to become an effective benchmark for assessing other homogenization theories and extended to other homogenization problems (such as thermal expansion coefficient) in the future.

8:00 PM - TC06.15.23

Phase Transformation and Plastic Deformation in an AFM Silicon Tip under Load

A silicon sensor-tip is an integral part of an atomic force microscope (AFM) used not only as an imaging stylus but also as a probe for assessment of mechanical properties of materials. In the AFM methods dedicated to mechanical characterization, the AFM tip has to physically touch the sample and withstand sometimes significant pressures. The basic principle of these methods is based on the evaluation of the elastic tip-sample interaction. However, high static loads applied to the tip in contact with materials of similar mechanical properties as silicon or greater may result in phase transformation in the silicon tip, its plastic deformation, and/or fracture.Atomic force acoustic microscopy (AFAM) methods utilize resonance frequencies of an AFM cantilever vibrating while in contact with the sample surface to determine the tip-sample contact stiffness. In the single-point spectroscopy mode, the contact stiffness k* is determined as a function of increasing and decreasing load. Maintaining reversible character of the load-unload stiffness curves ensures elastic tip-sample interaction and minimal tip wear. In our studies, we pressed several AFM tips against reference samples at ever increasing static loads and observed how the character of the load-unload stiffness curves changes from elastic to hysteretic. High resolution transmission electron microscopy (HRTEM) images helped us to explain some of the characteristic features occurring in the load-unload curves. For example, a HRTEM image of the tip used only to obtain reversible stiffness curves showed a crystalline tip coated with a thin layer of native oxide and free of any visible deformations or defects. Additional features such as stacking faults and partial dislocation formation were visible in images obtained for tips yielding hysteretic load-unload curves with very characteristic plunge of the contact stiffness during load (“softening”) followed by its strong rise during the unload (“stiffening”).We observed an additional type of load-unload curves, where the load stiffness formed at least one plateau while the unload stiffness showed stiffening in its initial phase. HRTEM images obtained for the AFM tips yielding this type of hysteretic curves did not show any visible structural changes or presence of the amorphous phase. We presumed that the plateau formation during the loading phase is caused by a phase transformation occurring in the silicon tip. To confirm our hypothesis, we added to our AFAM set-up an additional system utilizing the concept of the spreading scanning resistance mode. With help of simultaneously measured stiffness and effective resistance of the system as a function of the increasing and decreasing load we were able to identify formation of distorted diamond structure and metallic phase Si-II upon loading as well as transformation to Si-III and Si-XII upon unloading.

8:00 PM - TC06.15.24

A Novel Approach to Intermittent Nano-Indentation and Indentation Size Effect

Dynamical methods offer a natural platform to describe instabilities. Recently, we developed a dislocation dynamical model to explain load drops in displacement controlled(DC) experiments and displacement jumps in load controlled(LC) experiments with a view to develop an alternate theoretical approach to simulations. We set-up a system of coupled nonlinear evolution equations for the mobile and forest dislocation densities. We include nucleation, multiplication, and propagation threshold mechanisms for mobile dislocations apart from other well known dislocation transformation mechanisms between the mobile and forest dislocations. The evolution equations are coupled to relevant equation defining the loading condition. The model predicts not just the generic features of nano-indentation, but numbers that closely match experiments. For instance, the model predicts the existence of an initial elastic branch followed by load drops in the case of DC experiments. The approach has been extended for the conceptually more difficult case of LC mode experiments where there is a total absence of any kind of simulations. Our approach allows us to adopt experimental parameters such as the indentation rate, the geometrical quantities defining the Berkovitch indenter etc. We first identify specific dislocation mechanisms contributing to different regions of the F-z curve as a first step for obtaining a good fit to a given experimental F-z curve. This is done by studying the influence of the parameters on the model F-z curves. The model predicts all the generic features of nano-indentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes, and residual plasticity after unloading for a range of model parameter values. Further, optimized set of parameter values can be easily determined that gives a good fit to the experimental force-displacement curve for Al single crystals of (110) orientation and residual plasticity after unloading. The approach has been extended to explain the characteristic features of indentation size effect(ISE). For instance, our model predicts that a plot of the square of indentation hardness as a function of the inverse of the indentation depth is linear up to a tenth of micrometer but shows a tendency for saturation for nano-meter scales. We are able to get good fit to the hardness data for the polycrystalline Cu and single crystals of Ag. The approach offers a novel way of describing ISE based on dislocation evolution equations and does not require information about dislocation densities from other sources. The approach also provides insights into several open questions.

8:00 PM - TC06.15.25

Study of Carbon Nanotube Films Conductivity Dependence on the Morphology for Applications in Stretchable Electronics

Sensors, based on the unique properties of novel nanomaterials (e.g. nanoparticles, nanotubes, nanowires, thin films) have been of the enhanced interest last time due to their strain sensing characteristics. For example, strain sensors comprised of carbon nanotubes (CNTs), serve as good alternatives for developing new sensors. The electromechanical properties of these strain sensors exhibit excellent characteristics compared to the traditional ones due to outstanding combination of plasticity and electrical properties.Since the pioneering works of Whitesides and co-workers at Harvard University on thermal induced buckling of metal thin films on elastomeric substrate of polydimethylsiloxane (PDMS) appeared, a great interest in applications to such buckling electronic is arisen. Mechanical buckling method was demonstrated to be a promising approach to realize stretchable ‘wavy’ structures on the elastomeric substrates. Most brittle inorganic materials could buckle and become stretchable with this approach. This relaxation leads to the spontaneous formation of highly periodic, wavy structures, which can be not only bent, but also stretched and compressed. Another simple way to investigate properties of the CNT films by applying stretching tension is ordinary stretching of the samples on elastic substrates.Two types of CNTs: single-walled carbon nanotube films (SWCNT), obtained by aerosol synthesis and SWCNT films from commercial suspension (Tuball, OCSiAl), which were used in this study were directly transferred onto PDMS substrates. PDMS is the most widely used silicon-based organic polymer is optically clear, and, in general, inert, non-toxic, and non-flammable and has excellent elastic properties.Electrical parameters dependence of both film types on stretching with multiple stretching/relaxation cycles under different strain values were investigated experimentally in this study. For the study of mechanical properties, homemade device, which allowed us to study resistance dependence on stretching and fatigue tests (more than 10 000 cycles) was used. In addition, the mechanisms of stretching were studied by means of special stage loaded into scanning electron microscope (SEM), which allowed to visualize changes in the morphology of the film in real time (in-situ), Moreover, along with experimental data, theoretical studies were conducted: different computational methods for calculating the surface resistance (per unit area) of the film were used.

This work was supported by Skoltech NGP Program (Skoltech-MIT joint project).

8:00 PM - TC06.15.26

Nanotwins in Boron Carbide and Related Superhard Materials

Qi An 1 1 Chemical and Materials Engineering, University of Nevada, Reno, Reno, Nevada, United States

The twin structures and their roles in mechanical properties are extensively investigated and well understood for metals and alloys. However, for covalent solids, their structures and response to applied stress are not established. Here we characterize the nanotwins structures in boron carbide (B4C) and related superhard ceramics such as boron suboxide (B6O), and boron rich boron carbide (B13C2) using quantum mechanics (QM) simulations coupled with transmission electron microscopy (TEM). The “asymmetric twins” have been observed and characterized in B4C, which arises from the interplay of stoichiometry, atomic positioning, twinning, and structural hierarchy. While the negative interfacial energy in the twinned B6O leads to the discovery of new phases of τ-B6O. Then deformation responses of these nanotwins are examined by QM simulations showing the strengthening effects for B4C and softening effects for both B6O and B13C2, which are validated by nano-indentation experiments. The nanotwinned B4C is stronger than single crystalline B4C because the presence of twins suppresses the stress decrease as the B-C bond between icosahedral clusters breaks. However, the nanotwins in B13C2 and B6O decrease the strength of the perfect crystal because the failure mechanisms of B13C2 and B6O do not involve the B-C bond breaking between icosahedra.

8:00 PM - TC06.15.28

Cold-Drawing of Multimaterial Fibers and Films to Produce Micro- and Nano-Scale Particles and Structures

Polymer cold-drawing is a process in which applied stress along the fiber axis causes the polymeric chains to orient themselves, reducing the diameter of a drawn fiber (or thickness of a drawn film). Cold-drawing has long been employed in the industrial production of high-strength fibers, such as polyester and nylon threads. Here we show that in a multimaterial fiber composed of a brittle core embedded in a cladding that undergoes cold-drawing, surprising new phenomenon arises: as the neck propagates along the fiber the brittle core fractures predictably, producing uniformly sized rods along meters of fiber. Furthermore, by using a stack-and-draw method to produce fibers with a high density of cores, the fragmentation process can be parallelized to produce large numbers of embedded micro- or nano-rods.Embedded, structured cores having arbitrary multimaterial transverse geometry – from sub-millimeter to sub-micron scales – are thus fragmented into a periodic train of rods held stationary in the polymer. Two options exist at this stage in the process: the rods may either be easily extraction via selective dissolution or, alternatively, self-healing of the brittle thread is possible via thermal restoration. This process is not limited to cylindrical threads embedded in a ductile, polymeric cladding. The cross section of the resulting rods is determined by the shape of the rod used as the core of the preform that produces the drawn fiber. We demonstrate this flexibility by producing multimaterial rectangular rods, multimaterial hollow cylindrical rods, and hollow triangular rods with a rectangular hole. The method is also applicable to composites with flat rather than cylindrical geometry, whereupon cold-drawing leads to the breakup of an embedded or coated brittle film into narrow parallel strips aligned normally to the drawing axis.The fragmentation effect is applicable to a wide range of materials in the core, as long as the core material is brittle enough that it does not stretch or itself cold-draw. We demonstrate this by using various materials in the core ranging from silicon, germanium, gold, and glasses, to silk, polystyrene, biodegradable polymers, and even ice. We observe, and verify through nonlinear finite-element simulations, a linear relationship between the smallest transverse scale and the longitudinal breakup period. This work may lead to dynamical camouflaging via a nanoscale Venetian-blind effect, scalable fabrication of micro- and nanoparticles with arbitrary cross sections, and large-area meta-surfaces for highly sensitive detection of pathogens.

High-entropy alloys (HEA) which comprise multiple principal elements have recently opened a vast area of research for scientists due to their novel properties over the conventional alloying systems. Superior mechanical performance at elevated temperatures and their excellent specific strength comparable to superalloys e.g. Inconel 718 or structural ceramics are related to their high configurational (statistical) entropy and sluggish diffusion of constituent elements. High-entropy effects promote more thermal stability in the system and maintain strength at high service temperatures up to even 85% percent of their absolute melting temperatures. However, the fundamental underlying mechanisms of controlling elevated temperature properties are still unknown because of the compositional complexity of the system and their interaction with each other. This even become more complicated when several phases are present and transform at higher temperatures. Here, we aimed to dynamically measure the nanoscale mechanical properties of single-phase CoCrFeNi and dual-phase AlCrFeNiTi high-entropy alloys at room and elevated temperatures up to 500°C using nanoindentation method. G200 laser heater nanoindentation capable of independent heating of both indenter and sample was utilized to locally investigate the elevated mechanical behavior of aforementioned structures. Ultra-fast and precise controlling on testing temperature using laser heater ensured the accuracy of the measurements and provided highest thermal stabilization and lowest thermal drift during measurements. High-resolution mechanical-properties mappings were also extracted from the high-speed nanoindentation over the large area to statistically address the effect of microstructural features e.g. grain boundaries, second phase precipitates, interfaces, etc. on mechanical properties at high temperatures. Fully integration of G200 leaser heater with continuous stiffness measurements (CSM) and high speed mechanical mapping (Express Test) methods allowed us to dynamically measure nanomechanical properties e.g. young modulus and hardness. Experiments were performed in environmentally controlled atmospheres to avoid high temperature damages to the sample and indenter tip and eliminate uncertainties related to oxidation and degradation issues. This papers studied microstructural stability upon heating and discussed governing hardening mechanisms at low and elevated temperatures for high-entropy alloys.

Experimental studies on interfacial properties of polymeric composites, such as glass transition temperature, showed that the interfacial strength was critical. Numerical studies could also predict interfacial properties based on interfacial strength. In this study, interfacial damping properties and interfacial strength of fiber based polymeric composites were measured by dynamic mechanical tests and micro-bond tests, respectively, with the objective of quantitative analysis for the correlation. Properties of polymers, varying from polar to non-polar, from amorphous to semi-crystalline, from low molecular weight to high molecular weight, were investigated. The results showed supportive predictions about interfacial damping properties of fiber based polymeric composites.

Crystalline, superelastic materials typically exhibit large recoverable strains through a reversible phase transition between martensite and austenite phases that are associated with twinning and de-twinning processes. Applicable to various alloys, ceramics and intermetallic compounds, this reversible phase transition serves as a general mechanism for superelasticity. In our recent work, a new mechanism for superelasticity has been observed in novel ternary intermetallic compounds having the ThCr2Si2 type structure. Of these materials, LaRu2P2 and CaFe2As2 micropillars both exhibit a reversible phase transition between tetragonal and collapsed tetragonal crystallographic phases under compression, allowing for up to 11% of recoverable elastic strain to be realized. Single crystal solution growth, in-situ micropillar compression, in-situ neutron diffraction and density functional theory (DFT) calculation were used to elucidate the unique superelasticity mechanisms. Additionally, we investigated the mechanical behavior of a hybrid phase CaΚFe4As4, a combination of CaFe2As2 and ΚFe2As2 superconductors. We found that CaΚFe4As4 can also accommodate large, reversible strains by pressure-induced crystallographic structural collapse from the parent tetragonal phase to a half-collapsed tetragonal phase. More notably, CaΚFe4As4 exhibits high temperature superconductivity, which can be switched on and off by transition to the half-collapsed phase below 35 K.

In this presentation, the differently structured ThCr2Si2 intermetallic compounds are compared and the presence of a cryogenic shape memory effect is discussed with the results of micropillar compression and DFT simulation. Furthermore, discussion of the potential applications of these materials as cryogenic linear actuators and as devices in which switching of superconductivity at operation in extremely cold environments with applied pressure is possible. We believe that our work can make a bridge between experiment and computation to provide a fundamental understanding of superelasticity of novel ThCr2Si2-structured intermetallic compounds and their hybrid structures.

Nanotechnology has been considered a breakthrough in advanced structural materials research. In the case of reinforced polymer composite materials, nanomaterials play a significant role in improving mechanical properties. Although CNT has the best mechanical properties, electrospun carbon nanofibers have been recognized as a cheap alternative for improving polymer composite properties. In terms of composite properties, prepregs give the best mechanical performance among the different techniques of the polymer composite materials fabrication process. Hence, Electrospun carbon nanofiber engineered prepregs have the potentials to be used in various structural applications. In the current study, the flexural behavior laminated composites fabricated using electrospun carbon nanofiber engineered prepregs is compared with controlled samples of composite fabricated using conventional prepregs. The preliminary results indicate a significant improvement in flexural performance of laminated composites when nanoengineered prepregs were used during the fabrication.

8:00 PM - TC06.15.33

Effect of Phase Transformation Pathways on High Temperature Silicon Hardness

Temperature and strain effects on silicon are important technologically, however there is as of yet no fully complete phase diagram correlating the deformation mechanism. While the transition to dislocation plasticity dominated regime above 350°C is well known, there is still a matter of ambiguity below that temperature, where phase transformations occur. The produced transformation zones can vary in type and magnitude, which is further complicated by the presence of limited plasticity and fracture. Using Raman spectroscopy and TEM cross section analysis of indentation zone, the phase transformation behavior of silicon during nanoindentation at high temperatures (25 to 600 °C) has been studied. The results showed an increase in hardness up to 200 °C followed subsequently by decreasing hardness with temperature. Raman maps of the indentation zone show spatial distribution of residual phases while TEM cross section studies identify Si-III/Si-XII phases at the center and a separate transformation region in the shear zone. In this intermediate temperature regime (<200 °C) Raman spectroscopy identifies silicon Si-IV phase at the edges of the indent. Here, an attempt is made to qualitatively define the ratio of hydrostatic to shear components and the resulting effect of shear zone transformation at elevated temperatures.

Elevated temperature Nanoindentation is a key technology for pushing the characterization of materials and microstructures at in-service conditions. Controlling temperature and temperature gradients during the high temperature indentation is critical to maintain thermal stability to levels lower than the desired measurement. Here, we present an analysis of the impact of tip material selection on thermal stability in nanoscale experiments as well as isolation of these effects from experimental data. Tungsten Carbide is examined as a high temperature tip material in comparison to traditional tip materials. Isothermal control of both the indenter tip and the sample is found to be key in minimizing thermal artifacts. We discuss the details and limitations of maintaining stability through tip material selection as well as maintaining isothermal conditions.

8:00 PM - TC06.15.35

A Nanoindentation Study of Deformation and Fracture Behaviors of Superelastic Intermetallic Compound CaFe2As2

We recently discovered superelasticity and shape memory effect in novel intermetallic compound CaFe2As2. This material exhibits superelasticity through reversible phase transition between tetragonal and collapsed tetragonal phases under c-axis compression. We confirmed from in-situ micropillar compression study that recoverable strain of c-axis deformation is nearly 13%, which is close to that of the state-of-the art crystalline shape memory materials. In this presentation, we show our further investigation on deformation and fracture behaviors of CaFe2As2 single crystal by using nanoindentation with a different tip radius.

Sn-solution was used to make a crystal with the exact 1:2:2 stoichiometry. We used transmission electron microscopy to characterize its microstructure and confirmed that our crystal is defect-free. Sharp Berkovich tip was used to understand plastic deformation and fracture behaviors. We observed from high resolution scanning electron microscopy that plastic deformation occurs at the first and second pop-ins, but lateral cracks are formed at the third pop-in. Density functional theory (DFT) was used to perform the cohesive traction separation simulation, which revealed that the potential fracture plane of lateral crack is a Ca-As layer with the weakest bond as well as the widest interplanar spacing. We also performed spherical indentation to characterize superelastic deformation. 45 mN indentation shows full elastic recovery, and Hertizian contact theory was used to estimate the phase transformation displacement. In addition, cyclic deformation with spherical tip was performed to characterize the damping effect and the fatigue resistance. We confirmed that this material has a great fatigue resistance even with large number of cycles of forward-backward transition while conventional shape memory alloy usually accumulates deformation damage during cyclic deformation. DFT simulation shows that collapsed tetragonal transition requires making and breaking As-As bond, which is similar with simple atomic stretching of conventional elastic deformation and is not associated with any dislocation processes and residual stress accumulation, which are usually responsible for fatigue damages in shape memory alloys. Thus, CaFe2As2 has a superior fatigue resistance.

Electrically driven actuation of high-strain elastomers can be harnessed in a wide range of applications including artificial muscles, deformable lenses, and other optical devices. Layered elastomer materials are attractive for such applications because the layer structure can impart new mechanical characteristics that cannot be achieved in the source polymers alone. Here, we report on a new class of multilayer elastomer films containing alternating layers of [poly(ethylene octene) (EO) and polyvinylidene fluoride terpolymer (THV)], both of which possess low Young’s moduli (Y < 40 MPa). We examine the strain response of these composites under electric field to elucidate how confinement in 30 to 150 nm thick layers greatly improves the electromechanical properties, and we present a model to validate and describe the changes that occur to the individual polymers’ mechanical properties as a result of confinement. The multilayer structure of the EO/THV films yields enhancement of a number of physical properties, including dielectric strength (EB), maximum axial strain (sz), and the number of actuation cycles attained before breakdown. 512-layer 75% EO/25% THV films achieve EB > 300 V/μm, sz > 4%, and can be cycled 3000 times without breakdown; all significant improvements over EO or THV films alone. Time domain electro-mechanical models were developed to describe the viscoelastic/plastic behavior of pure EO and THV films under the influence of applied electric fields, including the effect of partially restraining metallic electrodes. By applying the modeling formalism to the layered EO/THV system and comparing it to experiment, changes in constituent properties caused by the nano-layering were determined, including a 40% increase in Y and a fourfold increase in the viscoelastic time constant for EO in the layered system compared to the pure EO film. Under confinement, the THV modulus also increased sharply and its plasticity was nearly eliminated.

As many material systems such as engineered coatings, thin films and small-volume materials exhibit anisotropic mechanical properties, the present study was focused on developing new methods to extract the elastic and plastic properties of anisotropic materials. Using a combination of dimensional analysis and large deformation finite element simulations of triple indentations, a framework for capturing the indentation response of transversely isotropic materials is developed. By considering 4800 combinations of material properties, forward algorithms that predict the indentation response of materials and the reverse algorithms that predict the materials’ elastic and plastic properties from experimentally measured indentation responses are formulated for both longitudinal and transverse indentations. Issues of accuracy, reversibility, uniqueness, and sensitivity within the context of the indentation of transversely isotropic materials are addressed carefully. Using 1400 combinations of material properties, it is demonstrated that there is perfect reversibility between the material properties and their indentation responses as predicted by the forward and reverse algorithms. On average, the differences between the results of the finite element analysis and those predicted by the forward algorithms for longitudinal or transverse indentations are less than 1%, thus demonstrating the high accuracy and uniqueness of the forward analysis. For longitudinal and transverse indentations, the reverse algorithms provide accurate results in most cases with an average error of 3% and 6%, respectively. A sensitivity analysis with a ±2% variation in the material properties in the forward algorithm and ±2% variation in the indentation responses in the reverse algorithms demonstrated the robustness of the algorithms developed in the present study, with the longitudinal indentations providing relatively less sensitivity to variability in indentation responses as compared to the transverse indentations. The predictions of the method are applied to engineering materials and compared to experimental results for the case of isotropic systems. The indentation analysis is extended to the case of transversely isotropic thin film systems. With over 13,000 indentation simulations, a new nano-indentation based method for extracting the elastic and plastic properties of the transversely thin film systems is also developed.

Furthermore, algorithms that predict the hardness of transversely isotropic materials from experimentally measured indentation responses are formulated. It is demonstrated that hardness increases with an increase in the strain hardening exponent, the average yield stress and the average elastic modulus. Multiple materials with different combination of elastic and plastic properties can exhibit identical hardness values. Hardness tends to be higher for materials with plastic anisotropy and lower for materials with elastic anisotropy.

Nature has meticulously structured nacre, a bio-ceramic composite, in a way that exhibits significant mechanical toughness. The toughness of nacre inherits its bulk properties from structural mechanisms, interface and chemical interactions in multiple scales. The required work to fracture the abalone nacre is 3000 times more than that of the pure bio-ceramic present in the system. This demonstrates the significant role of the organic matrix within the structure of the composite. In nanoscale structures of nacre, aragonite crystal asperities confine a layer of matrix. In the proposed research, topological constrains and organic-mineral interactions are investigated in order to obtain a better understanding of nacre mechanical behaviors. This study relates the nanoscale mechanochemical behaviors with the macroscale properties of mechanically remarkable biomaterials employing a molecular dynamic approach.

The light, strong and durable characteristics of nanofiber-reinforced metal-matrix nanocomposites (MMNC) are attractive to a number of industries such as the aerospace and automotive industries. An adequate load transfer on the nanofiber-metal interface is essential in order to take advantage of the extraordinary mechanical properties of the reinforcing nanofibers, which is the core reinforcing mechanism in nanofiber-reinforced MMNC. Carbon nanotubes (CNTs) are one of the most promising reinforcing fibers for disruptive MMNC technology due to their ultra-strong, resilient and low density properties. However, the understanding of the interfacial load transfer on CNT-metal interfaces remains elusive, which has been a major scientific obstacle in the development of the CNT-reinforced MMNC technology. In this talk, we present recent experimental studies of the mechanical strengths of the interfaces formed by individual CNTs with aluminum (Al) matrices. The nanotube-metal interfacial strength was characterized by using in situ electron microscopy nanomechanical single-tube pull-out techniques. By pulling out individual tubes from metal matrices using atomic force microscopic force sensors inside a high resolution scanning electron microscope, both the pull-out force and the embedded tube length were measured with resolutions of a few nano-Newtons and nanometers, respectively. The nanomechanical measurements reveal the shear-lag effect in the load transfer on the CNT-Al interface and the effect of thermal processing on the CNT-Al interfacial strength. The research findings help to better understand the load transfer on the tube-metal interface and the reinforcing mechanism of the nanotubes, and ultimately contribute to the optimal design and performance of nanotube-reinforced metal nanocomposites.

Anisotropic frictional response and corresponding heating in cyclotrimethylene-trinitramine molecular crystal are studied using molecular dynamics simulations. The nature of damage and temperature rise due to frictional forces are monitored along different sliding directions on the primary slip plane, (010), and on non-slip planes, (100) and (001). Correlations between the friction coefficient, deformation and frictional heating are established. We have found that the friction coefficients on slip planes are smaller than those on non-slip planes. In response to sliding on a slip plane, the crystal deforms easily via dislocation generation and shows less heating. On non-slip planes, due to the inability of the crystal to deform via dislocation generation, a large damage zone is formed just below the contact area, accompanied by the change in the molecular ring conformation from chair to boat/half-boat. This in turn leads to a large temperature rise below the contact area.

With their increased strength and modulus compared to bulk material, polymer nanofibers are being developed as advanced multifunctional lightweight structures. However, polymers exhibit viscoelastic behavior which depends on time and temperature. Under applied stress at short time scales, entanglements act as springs, but at longer times, they unravel and molecules slide past each other. Time and temperature are equivalent because creep at the long-time scale can be simulated by the response at higher temperature. Here, we investigate the long-term creep (~years) of polyacrylonitrile (PAN) nanofiber using time-temperature superposition.We fabricated PAN nanofibers of 100 - 1000 nm diameter by electrospinning. In the reported literature to date, measurement of polymer nanofiber creep has been limited to several hours and room temperature due to challenges associated with stability of the experimental conditions. To address this issue, a surface micromachined MEMS device was designed and fabricated to perform nanotensile tests on individual nanofibers. The metrology is based on optical methods with displacement and force resolution of ~8 nm and 40 nN respectively.Using this new apparatus, creep and stress relaxation were assessed in tension using standard test protocols where stress or strain is kept fixed using feedback. Within the linear viscoelastic regime, the creep compliance can also be obtained with a variable stress input using the Boltzmann superposition principle. We have verified that the creep compliance values obtained using the variable stress protocol are consistent with values obtained using the standard test protocol as well as with literature values. At room temperature, a generalized Voigt-Kelvin model with a free damper fits the data well. Parameters of the creep compliance model that fits are elastic moduli of 1.47,15.4, 4.4 and 12.4 GPa, time constants of 1.65, 5.99 and 32.05 seconds and steady state viscosity of 3584.2 GPa*s. This variable stress protocol is best suited to our test device. To obtain a master curve, we will measure and report creep compliances using the variable stress protocol up to 100 degrees C in a temperature-controlled environmental chamber. This work will enable the prediction of long term behavior of polymer nanofibers as a function of temperature, time and gaseous environment.

8:00 PM - TC06.15.42

New Observations on the Brittle-to-Ductile Transition of Silicon by In Situ High Temperature Bending

Utilizing nanomechanical tools, the brittle-to-ductile transition (BDT) can be explored at small length scales. This is valuable for reliability of microelectronics and MEMS devices, but also allows study of specimens where the resulting competition between dislocations and cracks can be fully characterized. This is of value for comparing against modelling techniques to further the eventual goal of developing a more complete mechanism map for the BDT in silicon. Here, a new scheme for studying crack advance of sharp nucleated at temperatures from 25C to 600C is presented. This is conducted using doubly-clamped bending beams, which naturally arrest the crack thereby allowing easy post-mortem analysis. The self-arresting nature of this specimen geometry was also utilized to produce cracks from FIB notches, which was subsequently reshaped to remove the FIB notch. Compared against FIB notches, this removes the influence of gallium damage and allows a more accurate study of fracture toughness, as opposed to the notch toughness from the curved front of a FIB notch. It is observed that the apparent toughness stays relatively constant up to 300C, then rises to 600C coinciding with the observation of substantial plasticity. Furthermore, differences in crack paths, including branching is observed. This suggests the influence of dislocation shielding from dislocation arrays along the crack path is producing this increased toughness.

8:00 PM - TC06.15.43

Effect of Slip on Detwinning Behavior During Multi-Direction Compression of a Wrought Magnesium Alloy

The effect of slip on detwinning behavior of a wrought Mg alloy has been investigated,using a micro-grid method to measure the local strain.Two compression paths were designed. In one case compression was applied first along a direction promoting twinning,and then along a perpendicular direction promoting detwinning. In the second an intermediate deformation along a direction midway between these two directions was applied to promote basal slip. It is shown that a smaller strain is required for detwinning than for the initial twinning. Moreover, the interaction between basal slip and twinning accelerates the detwinning behavior.

8:00 PM - TC06.15.44

Study of Twin Wall Properties in Atomistic Simulations Using a Landau-Ginzburg-Based Interatomic Potential

Twin walls are common crystallographic defects that have a key role in the mechanical behavior of materials with non-cubic crystalline structure, such as ferroelectric materials or shape memory alloys. When stress is applied to such materials, they deform via a mechanism of twin wall motion. While continuum models for twin walls, such as the Landau-Ginzburg (LG) model, are employed to quantify static properties of twin walls, they are limited in describing twin wall motion under a driving-force.

In this work, we developed an LG-based interatomic potential and implemented it in atomistic simulations, in order to find a relation between the properties at the atomic level and the threshold stress of the material. The parameters of the potential were calibrated to fit the continuum model. The static twin wall width and energy were calculated at the atomic level and the interatomic potential was shown to reproduce the known results of the Landau-Ginzburg model.

We performed molecular dynamic simulations to determine the relation between atomic properties and threshold stress for twin boundary motion, as well as minimum-energy simulation techniques to describe the energy barrier for the twin wall motion. Finally, the atomistic model proposed here is shown to give a general description for twin wall motion in realistic materials.

Understanding the evolution of single-crystal nanoparticles as they react to the alterations of the surrounding physical-chemical environment allows to optimize the design of stable high-energy catalysts. However, in addition to the difficulties of performing accurate in-situ experiments, available techniques have limited access to these non-equilibrium processes either for their statistical reliability or their time-scale sensitivity. The present study aims at attaining fundamental insights in structural-microstructural related transitions of bimetallic nanoparticles by exploiting numerical simulations to rationalize available experimental data.Atomistic simulations corroborated by in-situ experimental data revealed characteristic features of the structural and microstructural evolution path, providing the bases for a targeted designed synthesis of optimal nanocrystallites for catalysis. The combined role of crystallite’s size and bimetallic alloy’s structures was explored in a phase-like diagram to isolate characteristic behaviors and guide further experimental investigations. Indeed, the suitable choice of environmental parameters and precursors allowed the controlled synthesis of complex crystallite’s microstructures. In addition, the data modelling gave access to detailed information on surface physical-mechanical properties (i.e., surface atoms energy, lattice distortion, structural order-disorder), eventually related to the local structure coordination.The distortion of the lattice bond energy provided by the combination of different chemical elements was directly exploited in the synthesis and controlled transformation of Core-Shell single crystal microstructures. The chemical activity of the particles was thus optimized by an effective computationally-based design of the resulting crystallites structural-microstructural features.

We use a combination of atom-swap Monte-Carlo (MC) and molecular dynamics (MD) to study the equilibrium structure and mechanical properties of Cu(1-x)Ag(x)|Ni multilayers with 6nm layer width. We find that Cu|Ni multilayers form a semi-coherent interface with a network of partial dislocations arranged in a regular triangular pattern. Ag is then alloyed to the Cu layers in order to tune the lattice misfit in a controlled manner. A combination of MC and MD was used to equilibrate Cu(1-x)Ag(x)|Ni with x=0%, 5% and 10% towards their thermodynamic equilibrium. We find segregation of Ag within the Cu layers at 300K and 600K and alloying of Cu and Ni at 600K in good agreement with the experimental binary phase diagrams of the Cu-Ag and Cu-Ni systems. The resulting structures were then sheared parallel and perpendicular to the normal of the bilayer interface. We generally find initial sliding at the Cu|Ni interface within the misfit dislocation network, followed by emission of partial disloc ations into the Cu layer, and finally an increase of flow stress with increasing Ag content. Additional calculations of biaxial tension that suppress sliding at the heterointerface show a similar trend. Hardening can be traced back to the formation of sessile stacking-fault tetrahedra whose density increases with the density of the interfacial misfit dislocation pattern that is controlled by the amount of Ag in the Cu layer.

8:00 PM - TC06.15.48

Density Change Controls the Pressure Sensitivity of Shear-Driven Amorphization in Silicon and Diamond

Covalent crystals can respond to severe deformation through the formation of amorphous shear bands. Despite the importance of shear-driven amorphization in the plastic deformation of brittle crystals like silicon and diamond, its underlying mechanisms are still under debate and even a qualitative understanding of its pressure dependence is presently lacking. Here we use molecular dynamics simulations to show that the pressure sensitivity of amorphization rates in silicon and diamond is linked to the density of the amorphous phase, which is larger than the density of the cubic diamond phase in silicon but lower in diamond. As amorphization progresses, atoms in the amorphous phase are pushed into the crystal in silicon, while atoms are pulled out of the crystal, into the amorphous phase in diamond. Since the density of the amorphous phase is linked to the density of the liquid and hence to the slope of the melt line in the pressure-temperature phase diagram, our work suggests a link between atomic-scale mechanical amorphization processes and well-documented equilibrium properties of solids.

Integration of Ge (or SiGe alloys) on Si is appealing in order to exploit the superior optical/electronic performances of Ge maintaining well-developed Si technology. The presence of a misfit strain arising from the lattice mismatch makes the introduction of defects during the growth unavoidable, leading the presentresearch toward the exploitation of vertical heterostructures (VHEs). In these systems an elastic stress relaxation can occur thanks to the presence of lateral free surfaces,reducing in this way the need for additional plastic relaxation [1]. Here we present a joint theoretical and experimental analysis of the dislocation distribution in graded epitaxial SiGe VHE. A continuum model for VHEs based on linear elasticity and including dynamics of dislocations was developed [2]. An energetic criterion is implemented to test the condition and eventually the optimal position for the insertion of dislocations. The coupling with a FEM solver is exploitedto allow for an exact numerical treatment of the stress fields in the presence of a complex distribution of free surfaces, correctly modelling the dynamics of dislocations. The results show that, by suitably under-etching the Si pillars, it is possible to reverse the sign of the Burgers vector of the dislocations. This helps explainingdifferences in the experimentally observed distribution of dislocations in SiGe crystals grown on vertical and under-etched pillars, leading to a strong reduction of defects in the latter case. The agreement between simulations and experiments is not simply qualitative: the predicted number of defects generated by multiplication processes in tall crystals is indeed fully consistent with the measured one [3].[1] M. Salvalaglio and F. Montalenti. J. Appl. Phys. 116, 104306 (2014) [2] O. Jamond, R. Gatti, A. Roos and B. Devincre, Int. J. Plast. 80, 19 (2016)[3] F. Rovaris, et al., submitted (2017)

Grain boundary sliding (GBS) is a potential deformation mechanism for superplastic or near-superplastic deformation in polycrystals and also for plasticity in nanocrystals. In this study, Two-dimensional (2D) GBS was achieved during a high-temperature shear test in oxide-dispersion-strengthed ferritic steel that exhibits an anisotropic 2D microstructure with largely elongated and aligned grains. The 2D GBS, dislocation slip and subsequent microstructural evolutions were examined using surface markers drawn by focused ion beam and electron back-scattered diffraction analysis before and after deformation in the near-superplastic state (region III). GBS was accommodated by transgranular dislocation activities initiating from grain protrusions or triple junctions and spreading into core areas, as described by the Ball–Hutchison model (A. Ball and M.M. Hutchison, 1969). The accommodation mechanisms were determined by the microstructural correlation between GBS-triggered stress concentration and available slip orientation, closely related to the angle θ between GBS and dislocation slip. When θ was small, GBS tended to be accommodated by a group motion of dislocations belonging to <111> {110} or <111> {112} slip systems (slip-band type). When θ was large, GBS tended to be accommodated by intragranular dislocation accumulation, which led to the development of sub-boundaries along {110} planes via dynamic recovery (sub-boundary type); the latter mechanism would be the origin of continuous dynamic recrystallization.

Amorphous structure aluminum oxide (Al2O3) films are used for various applications such as gas- and moisture-diffusion barriers. Al2O3 films deposited by atomic layer deposition (ALD) have good step coverage, high density and low surface roughness. However, these films contain more impurities and need longer processing time at lower growth temperatures. By Griffith’s theory, the fracture strength of brittle materials increases with decreasing thickness and reaches an ideal strength at a critical thickness. Also, metallic glass-metal nanolaminate composites had different mechanical behavior from metallic glass single layer, as reported by several authors: metal layers suppressed catastrophic failure of metallic glass. So here we look at the critical thickness of amorphous Al2O3 films, which are brittle materials, and the changes in the mechanical behavior of amorphous Al2O3 when it is laminated with the inorganic material. The push-to-pull tensile test, which requires simple sample preparation, was used here to measure mechanical properties of ultra-thin films. For sample preparation, we deposited Al2O3 films and other inorganic films on silicon substrate using ALD at low temperature (<100°C). Then the silicon substrate was selectively etched using XeF2 gas and thin-films was fabricated with dog-bone shape using focused ion beam (FIB). We analyzed effect of temperature and thickness on mechanical properties and found the critical thickness of Al2O3 film. We then made amorphous Al2O3-inorganic nanolaminate composites and measured mechanical properties by tensile testing by an in-situ system.

Metal/ceramic interfaces, being susceptible to delamination damage, often represent the weakest link in relevant engineering systems, such as ceramic coatings on machining tools and mechanical components. Weak tensile strength and shear strength of metal/ceramic interfaces limit the life of coated parts, and restrict the realization of the full potential for ceramic coatings technology. Recent work on the Ti/TiN interface by the present authors revealed that a plane of weak strength in both shear and tension exists in the Ti phase close to, but not at, the chemical interface. It was speculated that the existence of such a mechanically weak plane might not be an isolated instance, present only in the Ti/TiN system. Rather it may be a phenomenon common for multiple metal/ceramic combinations.In this presentation, we report on our recent research on mapping the mechanical properties of interfacial regions (including the chemical interfaces and within the metal phases close to the chemical interface) of several combinations of metal/ceramic systems using density functional theory calculations. We consider three metal phases (Ti, Cr and Cu) and three ceramics phases (TiN, CrN, VN) - a total of 9 combinations. For each combination, the coherent structure properties, such as the generalized stacking fault energy profile and the work of adhesion in the metal phase as well as at the chemical interface, are evaluated as functions of the distance from the chemical interface. In a similar fashion, the variation in the core structure of misfit dislocations, as quantified by the Nye tensor analysis, is examined as a function of their distance from the chemical interface. The Peierls barriers for the misfit dislocations are also calculated. Our results offer guidance to engineering of mechanically robust metal/ceramic interfaces.

In the present study, we investigate grain growth in nanocrystalline Zirconium (Zr) thin film using in-situ Transmission Electron Microscope (in-situ TEM) and molecular dynamics simulation. We deposited Zr thin film with <10nm grain size on a micro electro-mechanical system (MEMS) device. While electromigration is commonly viewed as a degradation phenomenon, we propose that low to moderate current density can, to the contrary, have conducive effect on defects and microstructures. Accordingly, we exposed the nanocrystalline specimens to various levels of current density in-situ inside a TEM. For current density of 8×105 A/cm2 we observed grain growth from <10 nm to more than 500 nm, without any visible damage. In addition to the experiments, we employed classical molecular dynamics simulation to investigate the underlying mechanism on grain growth. We used embedded atomic model (EAM) to describe the atomic interaction during the simulation. We model the electron flow by imposing an additional equivalent wind force on each atom. This equivalent electrical wind force enhances the atomic motion, which in turns facilitates the rearrangement of atomic position during the electrical current passage. Due to this atomic rearrangement at the grain boundary, grain size grows significantly. In our present study, we model three different size of grain 2.5 nm, 5.0 nm and 7.5 nm to investigate the grain growth mechanism. In addition to this, we also orient the grain at different angle such as 5○, 10○, 15○, 30○ and 45○ to investigate the effect of grain boundary orientation on grain growth. To evaluate the effectiveness of electrical annealing we calculate mechanical properties of as deposited, single crystal and electrically annealed polycrystalline Zirconium thin films. Our investigation shows that electrically annealed polycrystalline Zirconium film can recover failure strain as high as the single crystal does, whereas maximum stress for ‘as deposited’ Zirconium is found at much lower strain than the electrically annealed Zirconium film.

Modelling plastic deformation at the micro and nanoscale is a puzzling problem in material science. The presence of free surfaces and interfaces reduce the mean free path of dislocations and change the mechanical properties of materials.This change of properties cannot be taken into account neither in continuum plasticity models, because the discrete nature of plastic deformation cannot be neglected, nor in atomistic simulations, because the simulated volume is too small and the accessible time scale is limited.A reliable tool to model crystal plasticity at such scales is the Discrete-Continuum Model (DCM). The DCM is based on a coupling between 3D Dislocation Dynamics (DD) simulations and Finite Element (FE) method. The DD simulation code is in charge of the dislocation microstructure evolution, handling the discrete nature of dislocations, while displacement field and boundary conditions are handled by the FE simulation code.In particular, in the DCM framework, not only the interaction of dislocations with free surfaces and interfaces is recovered, but also plastic incompatibilities are naturally taken into account and complex loading, close to the experimental conditions, can be imposed to the simulated object.Here, the main features of the DCM are briefly presented, emphasizing the capability of studying the plastic deformation in nano-object using anisotropic elasticity. Then the mechanical behavior of single- and bi-crystalline Ni micro-samples is investigated, comparing the result of DCM simulations with experimental data. Finally, the intriguing possibility of computing X-ray diffraction maps, post processing simulation outputs, is highlighted.

8:45 AM - TC06.16.02

On the Origin of Slip Traces on Micro Samples under Cyclic Loading by Discrete Dislocation Dynamics Simulations

In microspecimens the dislocation microstructure evolution under fatigue loading is quite surprising. Bending experiments with single crystalline microbeams show the development of slip traces on the surfaces which are generated by dislocations leaving the specimen [1]. Under load, the dislocations are pushed in and over the neutral plane of the gradient and stabilized in a pile-up [2]. Upon unloading, the geometrically necessary dislocations disappear, but the dissolution path of the pile-up and the reason for associated permanent plastic deformation is less clear. By using Discrete Dislocation Dynamics simulations it is shown that the explanation for the formation of a slip trace during unloading and the almost dislocation free sample is an asymmetry between the backward and forward motion due to the mutual dislocation interaction [3]. Additionally, the imperfect geometry (e.g. taper) of the sample can introduce a bias due to the geometrically imposed change in length of dislocations during glide [3]. Based on this mechanism, the specimen can fail without storage of dislocations.

Three dimensional dislocation dynamics (DD) simulations are performed to make a critical assessment of two classes of strain gradient plasticity theories currently available for studying size dependent plasticity at the micrometer-scale. Non-incremental version of strain gradient plasticity theories is characterized by certain stress quantities expressed in terms of increments of strains and their gradients, whereas incremental version of theories employ incremental relationships between all stress quantities and the increments of strains and their gradients. From the previous work, two classes of theories showed marked difference in the prediction of the onset of plasticity under non-proportional loading. In this work, our DD results show that plastic flow could continuously occur even with passivation, which lead to severe hardening, without showing the elastic loading gap.

Three dimensional discrete dislocation dynamics methods (3D DDD) have been developed to explicitly track the motion of individual dislocations under applied stress. In practice, these methods are limited to relatively small plastic strains due to high computational cost associated with the interactions between large numbers of dislocations. This limitation motivates the construction of minimalistic approaches to efficiently simulate the motion of dislocations for higher strains and longer time scales. Two dimensional dislocation dynamics is among the existing minimalistic models which although fast and useful for planar problems, often is an over-simplification. In the present study, we propose a new minimalistic model for discrete dislocation dynamics that we call Geometrically Projected Discrete Dislocation Dynamics (GPDDD). In this approach, we simulate the motion of dislocation loops whose shapes are predefined such that the number of degrees of freedom does not increase as the loop expands. The simplest case is a rectangular loop comprised of two edge and two screw segments. We utilize our model to simulate single-slip loading of copper that can be directly compared to detailed 3D-DDD simulations using ParaDiS. A simulation using GPDDD requires ~100 times less degrees of freedom for a copper single slip-loading case which significantly reduces the computational time and memory, and we show that GPDDD is able to accurately capture the variation of the flow stress with strain rate.

Acknowledgements: R. B. Sills acknowledges Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

9:30 AM - TC06.16.05

From Discrete Dislocation Plasticity to a Statistical Model of Rough Surface Contact

It is well known that rough surfaces have asperities at a wide range of length scales; that contacting asperities may yield even under low loads; and that plasticity in crystalline metals is size dependent at length scales below tens of micrometers. The influence of size dependent plasticity on the contact and frictional properties, however, is still quite an open question.

Here we present a statistical description of rough surface contact that incorporates size dependent plasticity through a simple strain gradient plasticity theory. The intrinsic material length scale in the theory is obtained by fitting to two-dimensional discrete dislocation plasticity simulations of the flattening of a single asperity. The statistical model is an extension of the Greenwood-Williamson theory incorporating the interaction between asperities. The effectiveness of the statistical model is addressed by comparison with full-detail finite element simulations of rough surface contact using strain gradient plasticity. One of the most noteworthy results of the analysis is that contact force and total contact area remain linear, as for elastic materials, but with a proportionality factor that depends on the ratio of the rms roughness and the material length scale.

9:45 AM - TC06.16.06

Surface Roughening by Plastic Deformation in Amorphous and Crystalline Solids

Most natural and man-made surfaces appear to be rough on many scales. However, there is presently no unifying theory of the origin of roughness or the self-affine nature of the surface topography. One likely contributor to the formation of roughness is the process of mechanical deformation. In particular, the plasticity observed in mechanically driven amorphous and crystalline solids has been characterized by avalanches of irreversible rearrangements exhibiting a self-affine, power-law behavior. Using molecular dynamics we simulate the bi-axial compression of a metallic glass and a single crystal of gold. We extract the free surfaces during the deformation and characterize the roughness. Analysis of the power spectral density reveals Hurst exponents above 0.5 for both the amorphous and crystalline surfaces, and a similar self-affine scale invariance is found in the displacement correlations within the regime of plastic deformation.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Dislocations are simplistic objects: they are one-dimensional, their motion is constrained by the crystallography and they are surrounded by a stress field that decays with 1/r. However, once dislocations start to interact with themselves or with other microstructures, their collective behavior becomes extremely complex. This – despite the apparent simplicity of the individual object – is still far from completely being understood.

While today powerful microscopy and simulation methods reveal many important information about systems of dislocations, the analysis of such data or even the comparison among different methods is still lagging behind. Systematic data mining is still far beyond the possibilities of today's methodologies.

I will present a new unifying multiscale approach for characterizing and data mining of dislocation microstructures that works both for experiments and simulations and for very different length scales.

11:00 AM - TC06.17.02

Non-Invasive Tools for the Investigation of the Crystal Plasticity Transition—Nanoindentation and Machine Learning

Stochasticity and uncertainty has become the norm in microplastic deformation in crystals. I will describe efforts on identifying a probabilistic theory for the crystal plasticity transition using dislocation theory and experiments of nanoindentation. Two dimensional and three dimensional discrete dislocation plasticity simulations of nanoindentation are performed at a rather small depth, in order to probe the character of the intrinsic material plastic zone. High throughput nanoindentation experiments are performed in a variety of FCC metals (Ni, Cu, Al). We combine theory and experiments into a unified framework through utilizing machine learning methods. Through this work, we develop a framework where nanoindentation may be utilized as a microscopic probe of plasticity. Tensile pre-stress has an effect on nanoindentation hardness that can be detected in a statistical manner. Moreover, we also develop a machine learning framework to unveil the parent dislocation structure/strength distributions only through nano-indentation force-depth data or through simple Digital Image Correlation data.

11:15 AM - TC06.17.03

Modeling the Strength Statistics of Nanoindentation with Dislocation Dynamics

Nanoindention experiments reveal that the onset of plastic deformation occurs over a wide range of indenter loads when the indenter size approaches the mean dislocation spacing. Under these conditions, it has been shown that plastic deformation is due to the activation of pre-existing dislocations, rather than homogeneous dislocation nucleation. Quantifying the effect of dislocation density on the mean and variance of the critical indenter force has remained a challenge with current experimental capabilities. In the present work, we use discrete dislocation dynamics (DDD) simulations of indentation to quantify the effect of dislocation density on the mean and variance of the critical force to induce plasticity. Furthermore, we employ DDD to identify the underlying dislocation mechanisms that lead to the onset of plastic deformation and pop-in. To include free surface effects and the highly varying stress field induced by the indenter, we couple the bulk DDD simulator, ParaDiS, with a parallel finite element solver. Simulations are carried out for multiple randomly seeded dislocation configurations at several dislocation densities in order to obtain a statistically relevant measure on both the mean and standard deviation of the critical force. Our results suggest a power law scaling for both the mean and standard deviation with respect to the dislocation density, which is in good agreement with the limited experimental work on quantifying this effect.

11:30 AM - TC06.17.04

Computational Modeling of Rate Processes in Density Based Dislocation Dynamics

Theoretical efforts on coarse graining of dislocation ensembles have recently yielded the first prediction of self-organized dislocation patterns in crystals (S. Xia and A. El-Azab, Modell. Simul. Mater. Sci. Eng. 23 (2015) 055009). Such efforts aim to develop density-based models that capture the important physics of the underlying discrete dislocation system. While most work in this area focuses on spatial coarse graining, some recent works started to address the question of time coarse graining (S. Xia, J. Belak and A. El-Azab, Modell. Simul. Mater. Sci. Eng. 24 (2016) 075007). Here, we tackle this question by focusing on the rate processes associated with dislocation dynamics, e.g., cross slip and dislocation reaction rates. The concepts of marked point process and time series are used to analyze the statistical properties of these processes in both time and frequency domains. The statistical data required to perform this analysis is obtained using the method of dislocation dynamics simulation. The temporal correlations and correlation times of cross-slip and short-range reactions have been computed. It is found that the correlation time for cross-slip is the largest of all correlation times and, as such, it is considered here to be the coarse graining time-scale in continuum dislocation dynamics. Using this mesoscopic time-scale, a coarse grained stochastic representation of cross slip and dislocation reactions has been achieved. Cross slip has been implemented in continuum dislocation dynamics to predict the self-organized dislocation patterns. (This work is performed in collaboration with S. Xia.)

For a variety of materials, the introduction of notches can be used to investigate key features of their plastic behavior. In some amorphous materials, such as metallic glasses, notch strengthening (the average axial stress across the minimum section is greater than the average stress across a corresponding un-notched bar) is observed. There have also been reports of notch weakening and notch-insensitivity. Various interpretations of these behaviors have been proposed but the issue is not yet settled. Notched specimens have also been used to investigate the development and interaction of shear bands. This has implications for developing toughening strategies for materials having a shear band dominated failure mode. However, there is still no unified view of notch effects on shear band formation and interaction due, at least in part, to the difficulty in experimentally measuring the shear band free mechanical response of metallic glasses.

In this work, we study plane strain tension of amorphous un-notched and notched bars using a discrete shear transformation zone plasticity framework. The plastic deformation in this framework is accommodated by the formation of Shear Transformation Zones (STZs), which are modeled as Eshelby inclusions. Boundary value problem solutions are obtained by superposing analytical expressions for individual, isolated Eshelby inclusion fields in an infinite solid with a numerical solution to the linear elastic boundary value problem for the image fields that enforce the boundary conditions. The image problem is a standard linear elastic boundary value problem that is solved by the finite element method. Potential STZ activation sites are randomly distributed in the bars and constitutive relations are specified for their evolution. Results are presented for un-notched bars, for bars with a blunt notch and for bars with a sharp notch. The computed stress-strain curves are serrated with the magnitude of the associated stress-drops depending on bar size, notch acuity and details of STZ kinetics. Results show that shear bands (deformation bands) emerge upon deformation. In several cases, stress levels of the order of the theoretical strength occur within these bands. Effects of notch and specimen size on the stress-strain curves are explored. Depending on the STZ kinetics, both notch strengthening and notch weakening are obtained. The analyses provide a rationale for some conflicting findings regarding notch effects on the mechanical response of metallic glasses.

TC06.18: High Entropy Alloys

Session Chairs

Marc Legros

Eugen Rabkin

Thursday PM, November 30, 2017

Hynes, Level 2, Room 210

1:30 PM - TC06.18.01

Nanomechanics of Single Crystalline Micro- and Nanoparticles of Molybdenum

We employed the mechano-stimulated equilibration technique [1] to produce the micro- and nanoparticles of molybdenum exhibiting the equilibrium crystal shape. We found that the equilibrium crystal shape of molybdenum strongly depends on the oxygen partial pressure during the high-temperature annealing, which enabled us to produce both heavily faceted and nearly hemispherical single crystalline particles. We probed the compression strength of the obtained particles employing the in-situ nanoindentation with a flat diamond punch inside the scanning electron microscope. The particles deformed quasi-elastically up to the critical strain, at which they collapsed and transformed into the low aspect ratio discs. The rounded particles exhibited size-independent shear strength of about 17 GPa, whereas their faceted counterparts exhibited strong size effect, with the critical stress exponent close to -1. We discussed the exceptional, record-breaking strength of the particles in terms of their internal structure.

Mechanically robust nanoscale metallic materials are highly desirable in many miniaturized devices. However, the lack of strain hardening and controllable plasticity plagues such small-volume metals. Using Al-4Cu alloy as an example, here we show that a submicron-sized metallic material with ultrathin native oxide shell exhibits a high degree of deformation controllability, unprecedented strain hardening, size strengthening and toughness, in uniaxial tensile deformation. The metal/native oxide “composite” is easy to make, and the emergent properties extend well beyond the benchmark range known for metals in a normalized (i.e., dimensionless) strength-toughness plot. The origin of the combination of strengthening and plastic stability is that an intact ultrathin native oxide shell exerts a strong confinement on dislocation movement and annihilation, thereby breaking the envelope on dislocation storage and strain hardening achievable in small-volume metals. Ref. Acta Mater 126 (2017) 202-209

High-entropy alloys (HEAs) are entropy-stabilized solid solutions that consist of (near) equimolar multiple components of metallic elements. Generally, they have simple crystalline structures such as face-centered cubic (FCC) and/or body-centered cubic (BCC). Possibly owing to the lattice distortions caused by the random (disordered) distribution of multiple constituent atoms with different sizes, HEAs offer unprecedented properties such as an exceptional damage tolerance and an ultralow diffusivity. Although there has been growing interest in exploring the nanoscopic origin of the special properties of HEAs, however, the exact “distorted” lattice structure and “varied” interplanar spacing (under the balance of interatomic potentials of different neighboring atoms), and their correlation to the exceptional behavior remain unclear. Therefore in this study, FCC-series (Co-Cr-Fe-Mn-Ni) and BCC-series (Mo-Nb-Ta-V-W) of low-to-high-entropy alloys were prepared. The lattice structures and lattice parameters (pair distribution functions, PDFs) of these alloys were determined using systematic high-energy diffractions and compared to the balance lattices obtained by the Density Functional Theory (DFT) calculations. Nanoscopic mechanical behaviors in different crystal orientations (100, 110 and 111) were characterized by nanoindentations and the in-situ compression of micropillars in a scanning electron microscope (SEM). Structural analyses indicated that, compared to the low- and medium-entropy alloys, the FCC HEA exhibited an expanded unit volume (a decreased packing density from 0.74 to 0.735), while the BCC HEA exhibited a shrunk unit volume (an increased packing density from 0.68 to 0.685), along with broad PDFs, causing a diminished crystallographic anisotropy. Mechanical characterizations revealed that the diminished anisotropy yielded a diminished heterogeneity of elastic deformation, and the activated partial dislocations and multiple cross slips on the roughened slip planes led to a diminished heterogeneity of plastic deformation.

High entropy alloys is a new developing research field that has shown a lot of characteristic properties different from traditional one. This type of alloys are composed by nearly equi-atomic percentage of different elements, which is opposite to the traditional dilute solid solution concept. Thus the mechanisms and physical theorem behind need huge modification effort, from fundamental physical metallurgy such as configuration entropy, diffusion, stacking fault and twin to more application end such as tensile test, fatigue, fracture behavior, irradiation damage, oxidation, corrosion, electromagnetic wave absorption…etc.In this research, we will show how mixing entropy on face-centered cubic structure and hexagonal close packed structure affects the Gibbs free energy and phase stability in advance. This also can explain the origin of negative stacking fault energy observed in high entropy alloys and the excellent mechanical property of dual increasing on strength and ductility. Besides, whether high entropy alloys have sluggish diffusion phenomenon has been fiercely debated. We will discuss two approaches to this problem from the fundamental diffusion equation, and show how mixing entropy and lattice distortion play their roles in this. Brute force algorithm based on combinatorial mathematics theorem and crystal structure rotational point group would be used for mixing entropy calculation, as well as molecular dynamic simulation results to show the uncertainty range of vacancy formation and migration energy deviation.

Near-equiatomic multi-component high-entropy alloys (HEAs) have garnered increasing attention as they not only represent a new approach to alloy design but also have been observed to exhibit remarkable properties. In particular, the “Cantor” alloy CrMnFeCoNi has been shown to display an exceptional combination of strength, ductility and fracture toughness, i.e. damage tolerance, at room temperature which is only further enhanced at cryogenic temperatures. Despite this alloy being the most studied HEA to date, its resistance to crack growth under cyclic fatigue loading has not previously been characterized. To examine this material’s high-cycle fatigue behavior, disc-shaped compact-tension samples were monitored using back-faced strain compliance and tested under sinusoidal loading high-cycle fatigue at both ambient and cryogenic temperatures (293K and 198K) and subjected to cycling at different load ratio conditions (R = 0.1, 0.4, 0.7). At R = 0.1, the alloy shows a fatigue threshold increase of more than 30% with decrease in temperature from 293 K to 198K accompanied by an increase in the Paris exponent m. Examination of the fracture surfaces and crack paths indicate a transition from predominantly transgranular crack propagation at room temperature to intergranular-dominated failure at the lower temperature. Under increasing R-ratio testing, the fatigue thresholds follow a similar trend; a decrease in temperature was accompanied with an increase in both fatigue threshold and a changed in observed behavior of the fracture surfaces.

The influences of annealing on constitutive stress-strain behavior of nanocrystalline (nc) CoCrFeMnNi high-entropy alloy (HEA) were systematically investigated through a series of nanoindentation experiments using five different three-sided pyramidal indenters. The nc HEAs produced by high-pressure torsion (HPT) processing were subjected to annealing at 450oC for 1 and 10 h. Microstructural analyses using transmission electron microscopy (TEM) showed that three different nano-scale precipitates (NiMn-, FeCo-, and Co-rich phases) were formed in the primary single-phase matrix of nc HEA after annealing. The strain-dependent plastic flow of nc HEAs was estimated by using the indentation strain and constraint factor, revealing the significant strain softening in nc HEA which became more pronounced after annealing. TEM analysis of annealed nc HEA underneath the indenter suggested that the annealing-induced precipitates were dissolved during plastic deformation, implying that the softening behavior may be attributed to the deformation assisted dissolution of the hard intermetallic precipitates. The dissolution of these phases was rationalized by the destabilization of precipitates during plastic deformation due to the increase in interface energy. *This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (No. 2015R1A5A1037627 and No. 2017R1A2B4012255).

Impact damage is far reaching in today’s society from the battlefields to the touchdown line. Study of impact mechanics is crucial for the safety of millions of people facing different challenges. Contact sports, warfare, and any physical activity can subject the body to various degrees of injury. The soldier constantly needs knee protection from blunt impact to the ground; the football player needs protective headgear for collisions during tackling; the skateboarder needs to wear helmets and kneepads when practicing new tricks. However, despite ongoing technological innovation, contemporary armor designs are still not energy-absorbent enough to prevent all these types of injuries. Therefore, it remains necessary to develop future superior protective systems through novel, ingenious designs. The earliest and most prolific such designs can be seen in nature, where organic material is used for protection against predators in seashells, boxfish, and turtles, and a plethora of other organisms.

Conch shells, a type of seashell, are known for being one of the toughest biological body armors under the sea. However, the complexity of the conch shell three-tier hierarchical architecture creates a barrier to emulating its cross-lamellar structure in synthetic materials. Previous attempts at mimicking the conch shell laminate design do not capture the complexity of the material. In this work, we present a three-dimensional biomimetic conch shell prototype, which can replicate the crack arresting mechanisms embedded in the natural architecture.

Through an integrated approach combining simulation, additive manufacturing, and droptower testing, this study explicates the function of hierarchy in conch shell’s multiscale microarchitectures. Specifically, a combined theoretical and experimental analysis is adopted to investigate the attributes critical to its superior impact resistance. The overarching mechanism responsible for the impact resistance of conch shell is the generation of pathways for crack deviation, which can be generalized to the design of future protective apparatus such as helmets and body armor.

3:45 PM - TC06.19.02

Intrinsic and Pseudo Size-Effect in Scaffolded Porous Nanoparticles and Their Self-Assembled Ensembles

Scaffolded porous nanoparticles with well-defined diameter, shape and pore size have profound impacts in drug delivery, bone-tissue replacement, catalysis, sensors, photonic crystals and self-healing materials. However, understanding the interplay between pore size, particle size, and mechanical properties of such nanoparticles, specially at the level of individual nanoparticles and their ensemble states, is a challenge. Herein, we focus on porous calcium-silicate nanoparticles with various diameters - as a model system - and perform an extensive 900+ nanoindentations to completely map out their mechanical properties at three distinct structural forms from individual nanoparticles to self-assembled ensembles to pressure-induced assembled arrays. Our results demonstrate a notable “intrinsic size-effect” for individual porous nanoparticles around~200-500 nm, induced by the ratio of particle characteristic diameter to pore characteristic size (~3nm). Increasing this ratio results in a brittle-to-ductile transition where the toughness of the nanoparticles increase by 120%. This size-effect becomes negligible as the porous particles form superstructures. Nevertheless, the self-assembled arrays collectively exhibit a pseudo size-effect while a pressure-induced compacted arrays exhibits no size-effect. This study will not only impact tuning properties of individual scaffolded porous nanoparticles, but will lay the foundation to create self-assembled superstructures leveraging porosity and particle size to impart new functionalities.

Considering that no previous study has reported the indentation of a single, spherical and porous particle, our nanomechanical characterization will have an impact on a diverse range of industrial fields, where porous spherical particles are actively exploited. Examples include solid phase extraction, catalysis and drug-delivery.

4:00 PM - TC06.19.03

Transition from Inhomogeneous to Homogeneous Deformation by Tailoring Energy State in Metallic Glasses

As a kind of non-equilibrium materials, metallic glass can attain a wide range of energy states, where the short/medium range order and mechanical behavior could vary. However, metallic glasses usually have relative low energy states due to the limitation in the preparation methods, and the deformation of this kind of metallic glasses is highly localized in a narrow region, i.e. shear band. In this presentation, we will show that there is a transition from inhomogeneous to homogeneous deformation in metallic glasses when their energy state has been raised. Our experiments of nanoindetation and micro-compression indicate that with increasing energy state, the hardness and Young’s modulus decrease, and the pile-up disappears. Furthermore, at a critical value of energy state, shear band disappears, and metallic glasses undergo homogeneous deformation under uniaxial compression. The present work reveals the correlation between the deformation behavior and energy state in metallic glasses, which will help us to understanding the deformation mechanism of metallic glasses, even disordered systems.

Metallic glasses (MGs) possess remarkably high strength but nearly negligible tensile ductility due to the formation of catastrophic shear band. Purposely enhancing the inherent heterogeneity to promote distributed flow offers new possibilities in pushing the boundary of ductility accessible to monolithic simple phase MGs. Here, we report the effect of spatial heterogeneity of elasticity, resulting from the inherently inhomogeneous amorphous structures, on the deformation behaviors of Cu-Zr MGs, ductility in particular, by using multi-scale modeling methods. On the one hand, the local shear moduli of the MG follow a Gaussian distribution as characterized by molecular dynamics simulation; on the other hand, we employ the mesoscale shear transformation zone dynamics (STZ) model to investigate the deformation mechanism in various MGs by tuning their spatial correlation, i.e., correlation length, of local shear moduli. Two different shear band formation mechanisms have been unraveled by varying the nanoscale elastic elasticity. Additionally, a critical spatial correlation length of elastic heterogeneity is identified for the MG achieving the best tensile ductility. This discovery is significantly important to fundamentally understand the role of spatial heterogeneity in the deformation behaviors of MGs, and can facilitate the design of ductile monolithic MGs via tuning the inherent heterogeneity. Supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Three dimensional assemblies of polymer-grafted nanoparticles (PGNs) are of current interest for a wide array of mechanical and electrical applications. The areal grafting density of the polymer chains on the nanoparticle surfaces, and the molecular weight of the grafted polymers, determine the resulting interparticle spacing and volume fraction of the polymer and nanoparticle constituents. Because the polymer is chemically grafted to the particle surface particle-particle contacts are eliminated, as opposed to a physical blending process in which nanoparticle agglomeration is typically observed. The mechanical properties of PGN thin film assemblies are therefore dominated by the polymer-polymer entanglements and the ordering in the nanoparticle assembly. Previous studies of PGN thin films have largely relied on nanoindentation, due to the challenges of preparing sub-micron testing specimens. The emergence of microelectromechanical systems (MEMS) techniques for fabricating devices and microscale specimen preparation using focus ion beam (FIB) allows for in-situ study of failure process of PGN assemblies. Ex-situ tests have shown that crack processes dominate deformation in nanoscale PGN assemblies with low grafting density, but crazing occurs for assemblies with high grafting density. These correlations between PGN architecture and subsequent failure mode refines the PGN design space for the synthesis and fabrication of assemblies with defined mechanical properties. In this study fundamental processes of craze initiation, the effect of film thickness, and the role of silica nanoparticles are examined in polystyrene-silica PGN assemblies synthesized using the grafting-from method. In-situ mechanical tests of PGN films examined the role of film thickness at fixed strain rate by simultaneous collection real time strain-stress data during SEM imaging. The yield stress was found to be film thickness dependent, and individual stress-strain events were correlated in real-time with imaging of the craze/cracks widening and propagating across the PGN films.

Polymers exhibit fascinating size-dependent phenomena that can include stiffening or softening depending on the nature of the polymer and confinement. However, the structure-property relationships that govern confined polymers are difficult to unambiguously determine due to unavoidable interactions with other materials. For example, the mechanical properties of supported thin polymer films are challenging to quantify due to the mechanical influence of the support. Here, we present a combined computation and experimental method to measure the true mechanical properties of thin polymer films. First, we describe our recent work exploring the modulus of thin thermoplastic polymer films.[1] Through extensive finite element simulations, we determine a correction to the Hertzian contact model that depends upon a dimensionless film thickness and the Poisson’s ratio of the material of interest. Using this correction, we experimentally extract the thickness-dependent modulus of three supported glassy polymers with thicknesses using an atomic force microscope (AFM). We found that the modulus of all three sub-100 nm polymer films was smaller than that of the bulk. While the observed behavior of these glassy polymers matches results found by other methods, softer elastomeric materials exhibit categorically different behavior in which they stiffen upon confinement. To explore this, we present results from coupled computational-experimental studies of elastomers in which a drastic stiffening is consistently observed. In addition to elastic properties of polymers, this approach can be applied to unambiguously determining the failure mechanics of polymer films. Thus, we conclude by describing progress in determining yield stress in thermoplastic and thermosetting polymer thin films. Taken together, this coupled experimental-computation approach has the potential to shed new light on size-dependent phenomena in polymers.

In the present study, we investigate grain growth in nanocrystalline Zirconium (Zr) thin film using in-situ Transmission Electron Microscope (in-situ TEM) and molecular dynamics simulation. We deposited Zr thin film with <10nm grain size on a micro electro-mechanical system (MEMS) device. While electromigration is commonly viewed as a degradation phenomenon, we propose that low to moderate current density can, to the contrary, have conducive effect on defects and microstructures. Accordingly, we exposed the nanocrystalline specimens to various levels of current density in-situ inside a TEM. For current density of 8×105 A/cm2 we observed grain growth from <10 nm to more than 500 nm, without any visible damage. In addition to the experiments, we employed classical molecular dynamics simulation to investigate the underlying mechanism on grain growth. We used embedded atomic model (EAM) to describe the atomic interaction during the simulation. We model the electron flow by imposing an additional equivalent wind force on each atom. This equivalent electrical wind force enhances the atomic motion, which in turns facilitates the rearrangement of atomic position during the electrical current passage. Due to this atomic rearrangement at the grain boundary, grain size grows significantly. In our present study, we model three different size of grain 2.5 nm, 5.0 nm and 7.5 nm to investigate the grain growth mechanism. In addition to this, we also orient the grain at different angle such as 5○, 10○, 15○, 30○ and 45○ to investigate the effect of grain boundary orientation on grain growth. To evaluate the effectiveness of electrical annealing we calculate mechanical properties of as deposited, single crystal and electrically annealed polycrystalline Zirconium thin films. Our investigation shows that electrically annealed polycrystalline Zirconium film can recover failure strain as high as the single crystal does, whereas maximum stress for ‘as deposited’ Zirconium is found at much lower strain than the electrically annealed Zirconium film.

9:00 PM - TC06.20.02

Nanotwins in Boron Carbide and Related Superhard Materials

Qi An1 1 Chemical and Materials Engineering, University of Nevada, Reno, Reno, Nevada, United States

The twin structures and their roles in mechanical properties are extensively investigated and well understood for metals and alloys. However, for covalent solids, their structures and response to applied stress are not established. Here we characterize the nanotwins structures in boron carbide (B4C) and related superhard ceramics such as boron suboxide (B6O), and boron rich boron carbide (B13C2) using quantum mechanics (QM) simulations coupled with transmission electron microscopy (TEM). The “asymmetric twins” have been observed and characterized in B4C, which arises from the interplay of stoichiometry, atomic positioning, twinning, and structural hierarchy. While the negative interfacial energy in the twinned B6O leads to the discovery of new phases of τ-B6O. Then deformation responses of these nanotwins are examined by QM simulations showing the strengthening effects for B4C and softening effects for both B6O and B13C2, which are validated by nano-indentation experiments. The nanotwinned B4C is stronger than single crystalline B4C because the presence of twins suppresses the stress decrease as the B-C bond between icosahedral clusters breaks. However, the nanotwins in B13C2 and B6O decrease the strength of the perfect crystal because the failure mechanisms of B13C2 and B6O do not involve the B-C bond breaking between icosahedra.

Grain boundary sliding (GBS) is a potential deformation mechanism for superplastic or near-superplastic deformation in polycrystals and also for plasticity in nanocrystals. In this study, Two-dimensional (2D) GBS was achieved during a high-temperature shear test in oxide-dispersion-strengthed ferritic steel that exhibits an anisotropic 2D microstructure with largely elongated and aligned grains. The 2D GBS, dislocation slip and subsequent microstructural evolutions were examined using surface markers drawn by focused ion beam and electron back-scattered diffraction analysis before and after deformation in the near-superplastic state (region III). GBS was accommodated by transgranular dislocation activities initiating from grain protrusions or triple junctions and spreading into core areas, as described by the Ball–Hutchison model (A. Ball and M.M. Hutchison, 1969). The accommodation mechanisms were determined by the microstructural correlation between GBS-triggered stress concentration and available slip orientation, closely related to the angle θ between GBS and dislocation slip. When θ was small, GBS tended to be accommodated by a group motion of dislocations belonging to <111> {110} or <111> {112} slip systems (slip-band type). When θ was large, GBS tended to be accommodated by intragranular dislocation accumulation, which led to the development of sub-boundaries along {110} planes via dynamic recovery (sub-boundary type); the latter mechanism would be the origin of continuous dynamic recrystallization.

Metal/ceramic interfaces, being susceptible to delamination damage, often represent the weakest link in relevant engineering systems, such as ceramic coatings on machining tools and mechanical components. Weak tensile strength and shear strength of metal/ceramic interfaces limit the life of coated parts, and restrict the realization of the full potential for ceramic coatings technology. Recent work on the Ti/TiN interface by the present authors revealed that a plane of weak strength in both shear and tension exists in the Ti phase close to, but not at, the chemical interface. It was speculated that the existence of such a mechanically weak plane might not be an isolated instance, present only in the Ti/TiN system. Rather it may be a phenomenon common for multiple metal/ceramic combinations.In this presentation, we report on our recent research on mapping the mechanical properties of interfacial regions (including the chemical interfaces and within the metal phases close to the chemical interface) of several combinations of metal/ceramic systems using density functional theory calculations. We consider three metal phases (Ti, Cr and Cu) and three ceramics phases (TiN, CrN, VN) - a total of 9 combinations. For each combination, the coherent structure properties, such as the generalized stacking fault energy profile and the work of adhesion in the metal phase as well as at the chemical interface, are evaluated as functions of the distance from the chemical interface. In a similar fashion, the variation in the core structure of misfit dislocations, as quantified by the Nye tensor analysis, is examined as a function of their distance from the chemical interface. The Peierls barriers for the misfit dislocations are also calculated. Our results offer guidance to engineering of mechanically robust metal/ceramic interfaces.

Amorphous structure aluminum oxide (Al2O3) films are used for various applications such as gas- and moisture-diffusion barriers. Al2O3 films deposited by atomic layer deposition (ALD) have good step coverage, high density and low surface roughness. However, these films contain more impurities and need longer processing time at lower growth temperatures. By Griffith’s theory, the fracture strength of brittle materials increases with decreasing thickness and reaches an ideal strength at a critical thickness. Also, metallic glass-metal nanolaminate composites had different mechanical behavior from metallic glass single layer, as reported by several authors: metal layers suppressed catastrophic failure of metallic glass. So here we look at the critical thickness of amorphous Al2O3 films, which are brittle materials, and the changes in the mechanical behavior of amorphous Al2O3 when it is laminated with the inorganic material. The push-to-pull tensile test, which requires simple sample preparation, was used here to measure mechanical properties of ultra-thin films. For sample preparation, we deposited Al2O3 films and other inorganic films on silicon substrate using ALD at low temperature (<100°C). Then the silicon substrate was selectively etched using XeF2 gas and thin-films was fabricated with dog-bone shape using focused ion beam (FIB). We analyzed effect of temperature and thickness on mechanical properties and found the critical thickness of Al2O3 film. We then made amorphous Al2O3-inorganic nanolaminate composites and measured mechanical properties by tensile testing by an in-situ system.

Achieving full control of the dislocation distribution during growth of semiconductor thin films is a key step in several applications (e.g. in optics or microelectronics), as dislocations can hinder the performances of devices. It is thus highly desirable to develop a detailed, quantitative understanding of processes such as nucleation, gliding, mutual interaction and multiplication, the final goal being to establish solid relations between typical growth parameters and the actual dislocation distribution. One of the main problems stems in the difficulties encountered when attempting to experimentally provide a complete characterization of the defects. TEM analysis are precious, but suffer for some major disadvantages, being demanding and time-consuming in terms of sample preparation, and obviously destructive. Furthermore, particularly in the low-misfit case where dislocations can be spaced by hundreds of nanometers, TEM usually leads to poor statistics.Recent developments in fast-scanning X-rays microscopy have opened up the possibility to record detailed tilting-angle maps [1,2], as caused by the dislocation distribution in the film and in the substrate. The technique is non-destructive and leads to good statistics.Here we present a theoretical procedure allowing one to extract from such maps, and from the knowledge of the residual strain R in the film, the position of individual dislocations. Based on the exact (within linear elasticity theory) expression of the tilting angle produced in the crystal by a dislocation, and by fixing the density of misfit segments based on the R value, we generate distributions of dislocations and compare the predicted and measured tilt map. By looking for local deviations between simulated and experimental maps, we further change the dislocation distribution until a good match is obtained. To make the procedure efficient we directly exploit the physics of the system: instead of sampling all possible dislocation positions in the simulation cell, we use dislocation dynamics simulations to evolve the system from a starting guess to a local equilibrium position.The procedure is here applied to Ge0.07 Si0.93 films grown on Si(001) by Chemical Vapor Deposition. Interestingly, we find clear evidence of multiplication, leading to misfit dislocations positioning deep in the Si substrate [3]. Such predictions are confirmed by TEM images.By comparing results obtained for films of different thicknesses, we can directly monitor the relative importance of multiplication on strain release as a function of the film thickness.The present approach can be conveniently combined with the continuum growth model of Ref.[4] in order to predict the film roughness.

We use a combination of atom-swap Monte-Carlo (MC) and molecular dynamics (MD) to study the equilibrium structure and mechanical properties of Cu(1-x)Ag(x)|Ni multilayers with 6nm layer width. We find that Cu|Ni multilayers form a semi-coherent interface with a network of partial dislocations arranged in a regular triangular pattern. Ag is then alloyed to the Cu layers in order to tune the lattice misfit in a controlled manner. A combination of MC and MD was used to equilibrate Cu(1-x)Ag(x)|Ni with x=0%, 5% and 10% towards their thermodynamic equilibrium. We find segregation of Ag within the Cu layers at 300K and 600K and alloying of Cu and Ni at 600K in good agreement with the experimental binary phase diagrams of the Cu-Ag and Cu-Ni systems. The resulting structures were then sheared parallel and perpendicular to the normal of the bilayer interface. We generally find initial sliding at the Cu|Ni interface within the misfit dislocation network, followed by emission of partial disloc ations into the Cu layer, and finally an increase of flow stress with increasing Ag content. Additional calculations of biaxial tension that suppress sliding at the heterointerface show a similar trend. Hardening can be traced back to the formation of sessile stacking-fault tetrahedra whose density increases with the density of the interfacial misfit dislocation pattern that is controlled by the amount of Ag in the Cu layer.

We propose a two-mechanism theory to estimate the pinning effect of coherent precipitates on grain-boundary (GB) migration in grain growth, taking into account the important effect of elastic misfit strain at the coherent interface. The topic has not been carefully addressed in the past and it is of high academic and industrial interest. It is shown in a qualitative theory that, depending on the relative importance of the elastic and the GB contributions to the total free energy, Zener type stabilization or a novel elastic energy induced stabilization may occur during the grain growth. It is found that the pinning is most effective in the crossover region between these two mechanisms. A phase-field-crystal model is used to numerically validate the theory. Relevant experiments and potential impacts on alloy design are also discussed. Our work sheds light on an important materials science problem regarding the role of the elastic energy from coherent precipitates in GB pinning and may provide further guidance to improve GB strengthening and alloy design.

9:00 PM - TC06.20.09

Understanding the Effect of Interface In FCC-BCC Multilayered Nanocomposites through In Situ Microfracture Bending Experiments

Cu-Nb and Al-Nb multilayered nanocomposites, both FCC-BCC, exhibit different mechanical behaviour due to their varying interfacial shear strengths and properties. To understand this effect in their fracture behaviour, in situ microfracture testing inside a scanning electron microscope was conducted. Microscale beams of Cu-Nb and Al-Nb PVD nanolayers were loaded in the manner of the three-point bending test with fixed ends. Video capture was done to observe the crack behaviour while the experiment was performed. Cu-Nb nanolayers was found to have fractured along the grain boundaries without any crack initiation whereas Al-Nb nanolayers showed some difference in its deformation mechanism before a crack initiated at the notch and propagated straight through the beam. Such insights would not have been obtained in an ex situ experiment. Our in situ study provided unique insights on the microstructural origins of this crack propagation behaviour. This knowledge could thus lead to new fracture limits in nanomaterials design through interface engineering and hence open up design space for the use of advanced nanomaterials in general in real world systems.

Understanding the evolution of single-crystal nanoparticles as they react to the alterations of the surrounding physical-chemical environment allows to optimize the design of stable high-energy catalysts. However, in addition to the difficulties of performing accurate in-situ experiments, available techniques have limited access to these non-equilibrium processes either for their statistical reliability or their time-scale sensitivity. The present study aims at attaining fundamental insights in structural-microstructural related transitions of bimetallic nanoparticles by exploiting numerical simulations to rationalize available experimental data.Atomistic simulations corroborated by in-situ experimental data revealed characteristic features of the structural and microstructural evolution path, providing the bases for a targeted designed synthesis of optimal nanocrystallites for catalysis. The combined role of crystallite’s size and bimetallic alloy’s structures was explored in a phase-like diagram to isolate characteristic behaviors and guide further experimental investigations. Indeed, the suitable choice of environmental parameters and precursors allowed the controlled synthesis of complex crystallite’s microstructures. In addition, the data modelling gave access to detailed information on surface physical-mechanical properties (i.e., surface atoms energy, lattice distortion, structural order-disorder), eventually related to the local structure coordination.The distortion of the lattice bond energy provided by the combination of different chemical elements was directly exploited in the synthesis and controlled transformation of Core-Shell single crystal microstructures. The chemical activity of the particles was thus optimized by an effective computationally-based design of the resulting crystallites structural-microstructural features.

9:00 PM - TC06.20.12

Study of Twin Wall Properties in Atomistic Simulations Using a Landau-Ginzburg-Based Interatomic Potential

Twin walls are common crystallographic defects that have a key role in the mechanical behavior of materials with non-cubic crystalline structure, such as ferroelectric materials or shape memory alloys. When stress is applied to such materials, they deform via a mechanism of twin wall motion. While continuum models for twin walls, such as the Landau-Ginzburg (LG) model, are employed to quantify static properties of twin walls, they are limited in describing twin wall motion under a driving-force.

In this work, we developed an LG-based interatomic potential and implemented it in atomistic simulations, in order to find a relation between the properties at the atomic level and the threshold stress of the material. The parameters of the potential were calibrated to fit the continuum model. The static twin wall width and energy were calculated at the atomic level and the interatomic potential was shown to reproduce the known results of the Landau-Ginzburg model.

We performed molecular dynamic simulations to determine the relation between atomic properties and threshold stress for twin boundary motion, as well as minimum-energy simulation techniques to describe the energy barrier for the twin wall motion. Finally, the atomistic model proposed here is shown to give a general description for twin wall motion in realistic materials.